MTA 98-364 Database Administration Fundamentals

Schema-scoped secure-able

(stored procedure)one that is created within a schema and can receive permissions through that schema.

index

(table scoped object and not securable) is a data structure that improves the speed of data retrieval operations on a database table. The disadvantage of indexes is that they need to be created and updated,.

JOIN statements

, which combine columns from multiple different tables. JOIN statements can be specified in either the FROM or the WHERE clause, but it is recommended that you specify them in the FROM clause.

two major reasons you might want to provide a view instead of enabling users to access the underlying tables in your database:

- Views allow you to limit the type of data users can access. You can grant view permissions in designated tables, and you can also choose to deny permissions for certain information. - Views reduce complexity for end users so they don't have to learn how to write complex SQL queries. Instead, you can write those queries on their behalf and hide them in a view.

A regular character uses how much storage?

1 byte, 256 possible characters.

Boolean stores what data type?

1 or 0, true or false

What 3 reasons should you consider using a clustered index?

1. Columns contain a large number of distinct values 2. Columns are accessed sequentially 3. Queries return large result sets

What steps should you take to create full-text searching?

1. Create a full text catalog in the database 2. Create a full text index on the column in question

What are three things true about views?

1. If a view definition contains the DISTINCT keyword, rows cannot be deleted through the view. 2. The WITH OBJECT IDENTIFIER clause is used to specify a top level (root) object view. 3. The OR REPLACE option is used to change the definition of a existing view without dropping and recreating.

What are 3 characteristics of a simple view?

1. It does not have any usage of SQL group functions or grouping of data. 2. DML operations are allowed on the view 3. It fetches data from one database table only

Table sizes

1MB - 1024 KB - 1 field - 165 bytes. a 100k table - = 16113.28125 kb(15.73563 mb)

How do you gather the size of a file?

1MB = 1024KB 1 record is 165 bytes - Multiply by amt of records.

Define Normal Form

1NF - Eliminate duplicative columns from the same table. -Create separate tables for each group of related data and identify each row with a unique column (the primary key). 2NF - Move repeating fields into a new table that contains a primary key, and relate it back to the old table using a foreign key. (requirement that each field value in a table is associated with only one row) 3NF - Eliminate columns not- transitively dependent (useless) on key 4NF - all in table are dependent on superkey 5NF - every join is implied by superkeys of the table

A unicode characters uses how much storage?

2 bytes, This allows 65,536 characters at almost any language.

Which normal form ensures that each attribute describes the entity?

2nf

What is the default length for the CAST function?

30

A Parameter name must begin with

@

DROP

A DDL statement used to remove objects, such as tables, views, stored procedures, and triggers , from a database.

Trigger

A SQL procedure that initiates an action when an event(INSERT, DELETE or UPDATE) occurs.

SQL injection

A code injection technique that exploits security vulnerabilities in the DB layer of an application.

Which column do you need to set a value for when issuing a INSERT statement?

A column of the uniqueidentifer data type. You need to use the NEWID() function in the values list to generate the value.

What is SQLCMD?

A command line application that comes with SQL and exposes the management features of SQL Server. It allows SQL queries to be written and executed from the command prompt.

What is the diffrence between an application role and a database role?

A database role is assigned to users

Mark works as a Database Designer for Reon Inc. He is assigned the task to create a database for the company. He issues the following query to create the database. CREATE DATABASE '24342' What will be the output of the query?

A database will be created.

What is a two-phase commit system?

A feature of a transaction processing system which enables DB's to be returned to the pre-transaction state if some error condition occurs. All databases are updated or none of them are.

What always returns a value but never updates data?

A function

What is the difference between a function and a stored procedure?

A function can be directly referenced in a select statement, and return only a single scalar or table vaule.

What is abstraction?

A method of coding that enables a user to focus on the coding rather than the syntax for a specific database API, allowing them to use generic methods of access as long as they have the additional codes or libraries which fill in the blanks.

Name 3 things which can be used to improve query performance

A primary key, a UNIQUE index and a CLUSTERED index

What is XQuery?

A query and functional programming language designed to query collections of XML data.

relational database

A relational database is similar to a hierarchical database in that data is stored in tables and any new information is automatically added to the table without the need to reorganize the table itself. Minimizes the amount of redundant data stored in the database. database can have multiple parents.

ADD

A reserved Keyword and cannot be used as a object name or column name in a table unless delimited. Add is used to add a new column in a alter table statement.

Subquery

A select statement that is nested within another SQL statement.

What is represented by a column in a well-designed relational database table

A single entity attribute

What is the inside-out approach to db design?

A type of bottom-up approach, the inside-out method begins with identifying a few important concepts then proceeds outward radially.

Character stores what data type?

A-z or 0-9(any digit or letter that math functions will not be applied to)

How would you set fillfactor?

ALTER INDEX ALL ON dbo.OrderDetails REBUILD WITH (FILLFACTOR = 60);

What command would you use to track changes to a table?

ALTER TABLE tablename ENABLE CHANGE_TRACKING

INSERT

Adds one or more new rows to a table or a view in SQL Server. Must contain a table or view name into which to insert the data. Can be used to insert explicit values into a table or to insert the result set returned by a query into a table.

What is the CHECK constraint?

Allows the admin to limit the types of data a user can insert into that column of the database.

Explicit transaction

An explicit transaction is one where all events of the transaction either happen together or they don't take place at all.

server-scoped securable

Available across all database instances(network)(LOGIN)

Partial Backup

Backup copy of only specific file groups.

Why is it a bad idea to let a foreign key contain a NULL value?

Because it may be impossible to verify the constraints if a foreign key consists of two or more columns if one of them is NULL.

db_denydatawriter

Blocks modifying data in any table in the database. Will override any object-level grant

What is the bottom up approach?

Breaking down the smaller components so that each describes a basic fragment.

What is DB prototyping?

Building a working model of the DB system in order to suggest improvements or add new features.

What command allows a Windows account to access SQL-Server?

CREATE LOGIN

Tables created using the ________ statement are used to store data.

CREATE TABLE

The core DDL statements are:

CREATE, DROP, ALTER

Setupadmin

Can configure linked servers, extend stored procedures and the start-up stored procedure. This is a Fixed server role

Diskadmin

Can create, alter, and drop disk files. This is a Fixed server role

Dbcreator

Can create, alter,drop, and restore databases

A stored procedure

Can modify the data in permanent tables and return multiple values as output parameters. It doesn't control how underlying data is physically accessed. Can return a value to the caller.

db_denydatareader

Can read from any table in the database, this deny will override an object-level grant

ALTER

Changes an existing object(changes the object definition)

UPDATE

Changes existing data in one or more rows in a table or view(changes the data in the table)

USE

Changes the database context

DBCC CHECKFILEGROUP

Checks all tables file group for any damage

Which 4 things always have a related data type?

Column, localvariable, expression and parameter.

Constraints are also referred to as ____ constraints.

Column; limitations or rules placed on a field or column to ensure that data that is considered invalid is not entered.

Good candidate for key columns in a non clustered index

Columns frequently uses as a query join conditions.

DROP LOGIN

Command will remove a windows user or group from the SQL server. This will not alter Windows its self

CREATE

Creates a SQL Server database object (table, view, or stored procedure)

Full-Text index

Creates a full-text index on a table or indexed view in a database. Only one full-text index is allowed per table or indexed view, and each full-text index applies to a single table or indexed view

Any ___ permission will always override a GRANT permission.

DENY

What language are triggers written in?

DML or DDL

Advantage of database over spreadsheet

Database can parse out redundant storage and information obtained from various relational spreadsheets.

DDL influences _____, while _______ influences actual data stored in tables.

Database objects, DML

What is one thing to consider when creating a view?

Database performance

Schema

Database s-scoped secure-able. Created within a database and provides a namespace and a security container for database objects. Each secure-able object must have a unique name

A ____ backup contains only the data that has changed since the last FULL backup.

Differential

DEFAULT

Ensures domain integrity by ensuring each record has a column value if one is not assigned.

What command do you use to invoke a stored procedure?

Execute

T/F: A single INSERT statement can be used to add rows to multiple tables.

False

T/F: Null is a valid constraint

False, NULL is not a constraint

For the CHAR data set, it is a _____ length and uses ___ bytes:

Fixed, N

MAX

Function returns the maximum value of an expression(IE: The maximum salary of all employees)

Implicit Transactions

Generates a continuous chain of transactions without stopping until completed

Grant Execute

Gives a database user or group(defined with a role) specific permissions to data objects. Uses ON clause preceding it.

What does ANSI SQL syntax refer to?

Grammar and rules governing the structure and content of statements.

An ___ ____ is the same thing as a CROSS JOIN with a WHERE condition:

INNER JOIN

What's the difference between an INNER join and an OUTER join?

INNER will only produce matching rows from both tables while OUTER will join all rows from both tables whether they match or not.

SQL server supports ____ conversions without using actual callout functions CAST or CONVERT

Implicit,(automatic)

CRUD

In computer programing, create, read, update and delete (CRUD)are the four basic functions of persistent change

An inline table-valued Function

Includes only a single SELECT statement. Can accept parameters.

A ____ backup contains only the data that has changed since the LAST backup.

Incremental

covering index

Index that includes all columns used in the query. Can optimize performance because the query can be generated from the index without accessing the actual tables involved in the query.

SQL Server utility can view what entities

Instances of SQL server, Data-Tier apps, Databse Files, Volumes

referential integrity

Integrity between tables using relationships.

OLTP(Online Transaction Processing) database

Is optimized to handle a large number of simultaneous updates additions and deletions.

Define 4NF

Isolate independent multiple relationships

Define 5NF

Isolate semantically related multiple relationships

Which of the following statements are true regarding the procedural data manipulation language?

It requires users to specify which data is needed and how to obtain it, it is a low-level DML, it requires users to know the data structure used in the db.

What is a fillfactor?

It specifies a percentage that indicates how much free space will be in the leaf level of each index page.

What does the IDENTITY constraint do?

It's used on the primary key to automatically start with 1 and auto-increment by 1.

What is a clustered index?

Large amount of selects on a table, create a clustered index on the primary key of the table. This index should be the most common column that is in your WHERE clauses for queries against this table. So if most of the time you search by primary key, then leave it as the default. But if you search by DateCreated or LastName most of the time on this table, then you might want to consider changing the clustered index to this column instead. Each table can only have 1 clustered index. (faster searching)

Which DB design process allows you to create a data model independent of a specific DBMS?

Logical DB design

What are the three types of files in SQL?

MDF = Primary data files, NDF = Secondary data files and LDF = Log files

The code DML statements are (5)

MIDUS: MERGE, INSERT, DELETE, UPDATE, SELECT

The MS database server that hosts relational databases is called _____

MS SQL Server

ALTER TABLE

Modifies a table definition by altering, adding, or dropping columns and constraints, reassigning partitions or disabling or enabling constraints and triggers.(cannot change IDENTITY constraint)

in a relational database, data is stored in

Multiple tables that are related using primary and foreign keys

Between statement

Must have AND; can only admit two expressions(between(10000 and 50000)) cant add another between(10000 and 50000) and (60000 and 90000)

What prefix must you have in front of a string to use Unicode?

N

Can you change the IDENTITY constraint of an existing column with an ALTER statement?

No

Non Clustered Index

Non-clustered indexes are not copies of the table but a sorting of the columns you specify that "point" back to the data pages in the clustered index. Add non-clustered indexes for queries that return smaller result sets

What is a non-clustered index?

Non-clustered indexes are not copies of the table but a sorting of the columns you specify that "point" back to the data pages in the clustered index. Add non-clustered indexes for queries that return smaller result sets. (Optimal Performance)

Graphical designers are _________ and are drag-and-drop given.

Object-Oriented

Deterministic function

One that returns the same predictable value when called with same nvarchar(3500). Meets requirements using least amt of storage. Supports multiple language input parameters.

MERGE

Performs insert, update, or delete operations on a target table based on the results of a join with a source table.

Which form of database design uses secondary storage media?

Physical database design

DENY

Prevents users of members of a role from performing an action on a secure-able object, such as a view, or stored procedure. even if permission is grated directly through membership in another role.

What 2 things speeds up data retrieval?

Primary key constraints and Clustered indexes

When do you pick the DBMS?

Prior to the Logical design phase.

All users are automatically members of the ______ database role.

Public

What does floating-point store in relation to data types?

Real numbers

The foreign key constraint is a ____ identifier.

Relationship; Enforces referential integrity

DELETE

Removes rows from a table or view(, which deletes data from within a table) Use for smaller databases

Name the two types of prototyping

Requirements and Evolutionary

Use the _____ command to recover data that was accidentally deleted by a user.

Restore

SELECT

Retrieves rows from the database and enables the selection of one or many rows or columns from one or many tables in SQL Server.

Aggregate Functions

Return a single value, calculated from values in a column. Ex. COUNT

Scalar function

Returns a single value, based on the input value of a single field(EX: UCASE - Returns the value of the field in all upper case).

STR

Returns character data converted from numeric data. The second parameter is for the total length of the output including the decimal point, signs, digits, and spaces. output is round as specified in the third parameter.

In order to use views, you must use the ______ T-SQL statement to show data from the tables.

SELECT

What is the correct way to select an avg of a column?

SELECT AVG("column name") FROM "table name"

What clause is required when issuing a UPDATE statement

SET

Table-valued function

Selects data and returns the results as a table. Cannot modify any permanent tables.

Name 3 levels of security supported by SQL Server

Server, Database and Table

How do you adjust indexes to reduce fragmentation from page splits?

Set the fillfactor to 60.

What is the UNIQUE constraint?

Specifically identifies which column should not contain duplicate values.

Database objects are divide into two categories:

Storage and Programmability

Database objects are divided into two categories:

Storage and Programmability

What's the most efficient way to delete all rows from a table?

TRUNCATE command; does not log each row

What happens if data is missing for a particular column when designing the INSERT SQL statement?

The INSERT statement uses the default value for the column.

to search strings in a column

The LIKE operator is used together with the % wildcard character

UPDATE statement

The function of the is to change data in a table or view

What does degree refer to?

The number of columns.

What is native auditing?

The process of extracting trails on a regular basis so they can be transferred to a designated security system where the database admins do not have access, this ensures a certain level of separation of duties and provides evidence that the audit trails were not modified.

What is a foreign key?

This is a column in one table that points to the primary key in another table(defines relationship between two tables). Ensures domain and referential integrity, not entity integrity. There is not a predefined limit for foreign keys.

What is TRANSACT-SQL

This is the primary means of programming and manageing SQL Server. When you use an SSMS to perform an action, it is using T-SQL commands in the background to do the work.

What is the primary key?

This uniquely identifies a column in the db. Has to have a unique constraint by default. Cannot be Null and data type must match. Ensures entity integrity(Uniquely id each row). Not always created as CLUSTERED indexes if one is already made.

What's the difference between time, date, datetime, datetime2, datetimeoffset and smalldatetime?

Time is the 24 hr clock without time zone awareness(Minimizes storage but can store time the includes hours, minutes, seconds but not fractional seconds.) Date is used to define 1/1 to 12/31 9999 Datetime is accurate to .00333 seconds(1/1 1753 to 9999) datetime2 is accurate up to 100 nanoseconds datetimeoffset includes daylight savings time smalldatetime(1\1 1900 to june 6, 2079)(2 bytes) does not keep track of seconds.(cant use neg numbers) Time(3) stores fractional seconds, however increases storage space. timestamp - is uses for row versioning, not to store time data.

IDENTITY

To automatically assign sequential integer values to a column

ORDER BY

To cause the result set to be sorted by the value in a specific column or columns. Not allowed in a CREATE VIEW Statement.

DBCC CHECKDALLOC

To check all pages in a DB are correctly allocated

COMMIT statement

To commit changes made within a manual transaction to the database.

ON DELETE CASCADE

To ensure all child tables are deleted inf the parent is deleted. This can be your FOREIGN KEY constraint.

Name 4 types of decomposition

Top-down, Bottom-up, Inside-out, Mixed

If you are querying the same table for two different things you'd use a....

UNION

A ____ will combine the results of two or more queries into a resulting set that includes all the rows belonging to the query:

UNION; without any duplicates. Duplicates require UNION ALL.

What are 6 constraint types?

Unique, Check(domain Integrity), Default(domain integrity), Not Null, Primary Key(entity integrity), Foreign Key(referential integrity)

How do you start a transaction?

Use BEGIN TRAN

If you have already defined a view on a specific table, then you add columns to that table, what must you do to the view?

Use the CREATE or REPLACE VIEW command to redefine it.

What is a default constraint?

Used to insert a default value into a column.

What is the top-down approach?

When a schema is created through a series of successive refinements, starting with the first schema.

What is the With Execute Owner clause?

When creating a stored procedure, this can be used to allow the person running the SP to have the same permissions as the person who owns the SP. This is better than granting SELECT to the user.

What is a self-reference?

When the foreign key refers to columns in the same table.

When is the best time to back up dynamic log files?

When the server is stopped.

%

Wild Card character(IE: WHERE lastname LIKE 'J%'); LIKE operator is used with %

What are the three methods for whicha user can be initially identified?

Windows user logon, Membership in a windows group, SQL server logon

ON DELETE NO ACTION

Would prevent a parent row from being deleted if it had a related child rows in another table.

DELETE syntax

You can identify and delete individual rows from the database

What would happen if you had a BETWEEN operator of BETWEEN 'D' and 'F'

You would get all values for D, E but not F. (MUST HAVE AND IN BETWEEN STATEMENT)

Check constraint

allows you to limit the types of data a user can insert into a the database and that they meet a certain condition.

UPDATE clause

allows you to modify data stored in tables using data attributes. Note: If you don't specify what records to update, all records or rows will be updated with the new value. To update another table use FROM clause. In this, SET specifies column to be updated.

Both the EXCEPT and the INTERSECT statements

are designed to return distinct values by comparing the results of two queries.

Exact numeric data types

are the most common SQL Server data types used to store numeric information.

primary drawbacks to using indexes

are the time it takes to build the indexes and the storage space the indexes require. Note: each table can have only one clustered index , data can be sorted in only one way.

Transact-SQL data types

bit, decimal and numeric. Transact-SQL data types that have a fixed precision and scale. money and smallmoney are Transact-SQL data types you would use to represent monetary or currency values.

serveradmin fixed server role

can Configure server -wide settings and issue SHUTDOWN command which immediately stops the SQL server.

RESTORE DATABASE command

can be used with a previously taken full backup to recover a database. you can add a NO RECOVERY clause if you plan to apply a transaction log of differential backup as part of your recovery process.

db_ddladmin

can issue DDL commands(statements)(create, alter,drop)

db_backupoperator

can perform backups, checkpoints, and DBCC commands, but not restores(only sysadmins can perform restores)

sysobjects

contains one row for each function, constraint or trigger created within a database. Indexes are not stored in this table(sysindexes).

sysindexes

contains one row for each index and table in the database.

Not null constraint

ensures that data is entered in to a cell(cant be blank).

DBCC CHECKDB

ensures the tables in the DB and the indexes are correctly linked.

You would include a HAVING clause in a query to:

filter the results grouped by a GROUP BY clause.

EXCEPT clause

gives you the final result set where data exists in the first query and not in the second dataset.

INTERSECT clause

gives you the final result set where values in both of the queries match by the query on both the left and right sides of the operand. • The number and order of the columns must be the same in all queries. • The data types must be compatible.

Data Types

huge numbers(way bigger than millions) = bigint Big numbers= int(-2,147,483,648 to 2,147,483,647)Can store dates YYYYMMDD(4 bytes) small numbers = smallin(-32768 to 32,767 however req 2 bytes of space) smaller numbers = tinyint(max value of 255 with minimum storage 1 byte) lots of text = varchar(expands as needed) YYYYMMDD(8 bytes) text containing Unicode symbols(diff language or symbols) = nvarchar currency = smallmoney(-214,748.3648 to 214,748.3647 (4 decimal places)) yes/no = bit(value of 0,1 or null) Float = precision and scale(best for finding percent %) decimal(3) - 5 bytes

Cartesian Product

includes a row for each possible combination of results from the other tables. the number of rows returned will equal the number of rows in one table multiplied by the number of rows in the other table.

When querying a database you can obtain faster results from properly ______ tables and views.

indexed

database index

is a data structure that improves the speed of data retrieval operations on a database table.

Function

is a database object that can accept values, perform a calculation and return the result to the caller. Can be directly referenced using a SELECT statement.

application role

is a database-specific role intended to allow an application to gain access regardless of the user.

hierarchical database

is similar to a tree structure (such as a family tree). Each parent table can have multiple children, but each child table can have only one parent.

Flat-type databases

is simplistic in design. They are two dimensional tables that are consisting to rows and columns. for fast searches.

view

is simply a virtual table consisting of different columns from one or more tables. Unlike a table, a view is stored in a database as a query object; therefore, a view is an object that obtains its data from one or more tables. View is a database object that can be used in SELECT queries just like a table. When a view contains a join, DML can be preformed but only a single base table.

Precision (p)

is the maximum total number of decimal digits that can be stored in a numeric data type, both to the left and to the right of the decimal point; this value must be at least 1 and at most 38. The default precision number is 18.

De-normailization

is the process of stepping back through the normalization steps until you get something that is "comfortable". Improves data retrieval performance.

WHERE

is used to obtain only subset of rows that fulfill criterion in the WHERE clause

SELECT statement query

lets you produce individual lists, but the result may be that you get all the information you need but in an individual list format.

The sa account is only used in

mixed-mode

A user-defined function

must contain a RETURNS clause and a RETURN statement.

composite primary key

occurs when you define more than one column as your primary key.

A regular character uses ___ bytes of storage, whereas a unicode character requires ____ bytes.

one, two

Heap

physical structure in non clustered index

RIGHT OUTER JOIN

preserves the unmatched rows from the second (right) table, joining them with a NULL in the shape of the first (left) table

Collation

refers to a set of rules that determine how data is sorted and compared. By default, SQL Server has predefined collation precedence. If you wish to override how data is being sorted, you must use a collation clause.

REVOKE

removes user access rights or privileges on the database objects granted using the GRANT statement, but does not prevent permissions from being obtained through membership in other roles.

What does the REPLACE function do?

replaces all occurrences of the second given string in the first string expression with a third expression(SELECT REPLACE('The best TTS Solution','s','z') - The bezt TTZ ..

Cross Joins

returns a result set that contains all the combinations of rows from two tables that do not have a column in common, also know as a Cartesian product. WHERE conditions should always be included.

Full Outer Joins

returns all rows from the left table (table1) and from the right table (table2).

STR Function does what?

returns character data converted from numeric data.

CHARINDEX function

returns the starting position of the specifies expression in a character string. The third parameter of the function is the character position to start searching for the first parameter in the second parameter. If starting ocation is a negative number, search starts at beginning of second parameter.

INNER JOIN

returns when there is at least one match in both tables.

Normalization

series of steps(or rules) called "forms" the more steps you take, the more normalized your tables.

SET NOCOUNT statement

shows line stating the number of rows "affected". This is off by default. This will suppress the '(1 row affected)

Databases are often put on database servers for what reason

so that they can be accessed by multiple users and provide a higher level of performance. One popular database server is Microsoft SQL Server.

System views belong to the ______

sys schema

The ___ role gives access to anything on the SQL server, while the ____ role gives full access to a specific database:

sysadmin, db_owner

Explicit conversions require you to use

the functions cast or convert specifically.

what happens when you connect two tables

the primary key is replicated from the primary to secondary table, and all the key attributes duplicated from the primary table are known as the foreign key.

PERSISTENT clause

to create a computed column that can be included in an index.

HAVING clause

to filter the results grouped by a GROUP BY clause. Can only reference aggregate functions or column that are also specified in the GROUP BY clause.

COLLATE clause

to specify sorting rules that are different than those used for other columns in the table.

Application design involves 2 important activities:

transaction design and interface design

SET clause

update columns with calculated values

.WRITE

used in a UPDATE statement when you need to update the values of columns that have a large object data type such as varchar(max) or varbinary(max) columns.

CREATE TYPE

used to create a user-defines data type or an alias data type.

CREATE INDEX statement

used to define a clustered or non clustered index.

WITH CHECK OPTION

used to ensure that no changes to the underlying data can be made through the view that would cause data no to conform to the definitions of the view.

WITH SCHEMABINDING

used to ensure that none of the tables references by the view are dropped or have their structure changes such that the view would not function properly. Cannot use asterisk in SELECT list., must specify column by name

spatial index

used to optimize operations that use spatial data types, such as geometry and geography data types.

OPTION

used to specify query hints that will allow for optimization of a query's execution.

Windows Authentication mode is superior to mixed mode because

users don't need to learn yet another password and because it leverages the security design of the network.

A bit is the T-SQL integer data type that can take a ___ of 1, 0 or NULL.

value

Inline table

valued function includes only a single SELECT statement.

A clustered index usually _____ performance when inserting data.

worsens, because it's constantly sorting it. Improves it for retrieving it though. Physical sort. Only 1 per table.

Some common examples of views include the following:

• A subset of rows or columns of a base table • A union of two or more tables • A join of two or more tables • A statistical summary of base tables • A subset of another view or some combination of views and base tables

three things you need to be sure to identify in your statement to form a proper SELECT query:

• Columns to retrieve • Tables to retrieve the columns from • Conditions, if any, that the data must satisfy

Views ensure the security of data by restricting access to the following data

• Specific rows of tables • Specific columns of tables • Specific rows and columns of tables • Rows obtained by using joins • Statistical summaries of data in given tables • Subsets of another view or subsets of views and tables