sql questions

What is a View?

A view is a virtual table which consists of a subset of data contained in a table. Views are not virtually present, and it takes less space to store. View can have data of one or more tables combined, and it is depending on the relationship.

What is an Index?

An index is performance tuning method of allowing faster retrieval of records from the table. An index creates an entry for each value and it will be faster to retrieve data.

What is the difference between Cluster and Non-Cluster Index?

Clustered index is used for easy retrieval of data from the database by altering the way that the records are stored. Database sorts out rows by the column which is set to be clustered index. A nonclustered index does not alter the way it was stored but creates a complete separate object within the table. It point back to the original table rows after searching.

What is the difference between DELETE and TRUNCATE commands?

DELETE command is used to remove rows from the table, and WHERE clause can be used for conditional set of parameters. Commit and Rollback can be performed after delete statement. TRUNCATE removes all rows from the table. Truncate operation cannot be rolled back.

What is a relationship and what are they?

Database Relationship is defined as the connection between the tables in a database. There are various data basing relationships, and they are as follows:. One to One Relationship. One to Many Relationship. Many to One Relationship. Self-Referencing Relationship.

13. What are all the different normalizations?

First Normal Form (1NF):. This should remove all the duplicate columns from the table. Creation of tables for the related data and identification of unique columns. Second Normal Form (2NF):. Meeting all requirements of the first normal form. Placing the subsets of data in separate tables and Creation of relationships between the tables using primary keys. Third Normal Form (3NF):. This should meet all requirements of 2NF. Removing the columns which are not dependent on primary key constraints. Fourth Normal Form (3NF):. Meeting all the requirements of third normal form and it should not have multi- valued dependencies.

What is the difference between IN and EXISTS?

IN: Works on List result set Doesn't work on subqueries resulting in Virtual tables with multiple columns Compares every value in the result list Performance is comparatively SLOW for larger resultset of subquery EXISTS: Works on Virtual tables Is used with co-related queries Exits comparison when match is found Performance is comparatively FAST for larger resultset of subquery

what are the different types of JOIN?

INNER JOIN (a.k.a. "simple join"): Returns all rows for which there is at least one match in BOTH tables. This is the default type of join if no specific JOIN type is specified. LEFT JOIN (or LEFT OUTER JOIN): Returns all rows from the left table, and the matched rows from the right table; i.e., the results will contain all records from the left table, even if the JOIN condition doesn't find any matching records in the right table. This means that if the ON clause doesn't match any records in the right table, the JOIN will still return a row in the result for that record in the left table, but with NULL in each column from the right table. RIGHT JOIN (or RIGHT OUTER JOIN): Returns all rows from the right table, and the matched rows from the left table. This is the exact opposite of a LEFT JOIN; i.e., the results will contain all records from the right table, even if the JOIN condition doesn't find any matching records in the left table. This means that if the ON clause doesn't match any records in the left table, the JOIN will still return a row in the result for that record in the right table, but with NULL in each column from the left table. FULL JOIN (or FULL OUTER JOIN): Returns all rows for which there is a match in EITHER of the tables. Conceptually, a FULL JOIN combines the effect of applying both a LEFT JOIN and a RIGHT JOIN; i.e., its result set is equivalent to performing a UNION of the results of left and right outer queries. CROSS JOIN: Returns all records where each row from the first table is combined with each row from the second table (i.e., returns the Cartesian product of the sets of rows from the joined tables). Note that a CROSS JOIN can either be specified using the CROSS JOIN syntax ("explicit join notation") or (b) listing the tables in the FROM clause separated by commas without using a WHERE clause to supply join criteria ("implicit join notation").

Given two tables created as follows create table test_a(id numeric); create table test_b(id numeric); insert into test_a(id) values (10), (20), (30), (40), (50); insert into test_b(id) values (10), (30), (50); Write a query to fetch values in table test_a that are and not in test_b without using the NOT keyword. Note, Oracle does not support the above INSERT syntax, so you would need this instead: insert into test_a(id) values (10); insert into test_a(id) values (20); insert into test_a(id) values (30); insert into test_a(id) values (40); insert into test_a(id) values (50); insert into test_b(id) values (10); insert into test_b(id) values (30); insert into test_b(id) values (50);

In SQL Server, PostgreSQL, and SQLite, this can be done using the except keyword as follows: select * from test_a except select * from test_b; In Oracle, the minus keyword is used instead. Note that if there are multiple columns, say ID and Name, the column should be explicitly stated in Oracle queries: Select ID from test_a minus select ID from test_b MySQL does not support the except function. However, there is a standard SQL solution that works in all of the above engines, including MySQL: select a.id from test_a a left join test_b b on a.id = b.id where b.id is null;

25. What are local and global variables and their differences?

Local variables are the variables which can be used or exist inside the function. They are not known to the other functions and those variables cannot be referred or used. Variables can be created whenever that function is called. Global variables are the variables which can be used or exist throughout the program. Same variable declared in global cannot be used in functions. Global variables cannot be created whenever that function is called.

Given the following tables: SELECT * FROM users; user_id username 1 John Doe 2 Jane Don 3 Alice Jones 4 Lisa Romero SELECT * FROM training_details; user_training_id user_id training_id training_date 1 1 1 "2015-08-02" 2 2 1 "2015-08-03" 3 3 2 "2015-08-02" 4 4 2 "2015-08-04" 5 2 2 "2015-08-03" 6 1 1 "2015-08-02" 7 3 2 "2015-08-04" 8 4 3 "2015-08-03" 9 1 4 "2015-08-03" 10 3 1 "2015-08-02" 11 4 2 "2015-08-04" 12 3 2 "2015-08-02" 13 1 1 "2015-08-02" 14 4 3 "2015-08-03" Write a query to to get the list of users who took the a training lesson more than once in the same day, grouped by user and training lesson, each ordered from the most recent lesson date to oldest date.

SELECT u.user_id, username, training_id, training_date, count( user_training_id ) AS count FROM users u JOIN training_details t ON t.user_id = u.user_id GROUP BY u.user_id, username, training_id, training_date HAVING count( user_training_id ) > 1 ORDER BY training_date DESC;

Table is as follows: ID C1 C2 C3 1 Red Yellow Blue 2 NULL Red Green 3 Yellow NULL Violet Print the rows which have 'Yellow' in one of the columns C1, C2, or C3, but without using OR.

SELECT * FROM table WHERE 'Yellow' IN (C1, C2, C3)

Given a table dbo.users where the column user_id is a unique numeric identifier, how can you efficiently select the first 100 odd user_id values from the table? (Assume the table contains well over 100 records with odd user_id values.)

SELECT TOP 100 user_id FROM dbo.users WHERE user_id % 2 = 1 ORDER BY user_id

45. How can you create an empty table from an existing table?

Select * into studentcopy from student where 1=2

What is the difference between single-row functions and multiple-row functions? What is the group by clause used for?

Single-row functions work with single row at a time. Multiple-row functions work with data of multiple rows at a time. The group by clause combines all those records that have identical values in a particular field or any group of fields.

43. What is the difference between TRUNCATE and DROP statements?

TRUNCATE removes all the rows from the table, and it cannot be rolled back. DROP command removes a table from the database and operation cannot be rolled back.

What is the difference between the RANK() and DENSE_RANK() functions? Provide an example.

The only difference between the RANK() and DENSE_RANK() functions is in cases where there is a "tie"; i.e., in cases where multiple values in a set have the same ranking. In such cases, RANK() will assign non-consecutive "ranks" to the values in the set (resulting in gaps between the integer ranking values when there is a tie), whereas DENSE_RANK() will assign consecutive ranks to the values in the set (so there will be no gaps between the integer ranking values in the case of a tie). For example, consider the set {25, 25, 50, 75, 75, 100}. For such a set, RANK() will return {1, 1, 3, 4, 4, 6} (note that the values 2 and 5 are skipped), whereas DENSE_RANK() will return {1,1,2,3,3,4}.

Assume a schema of Emp ( Id, Name, DeptId ) , Dept ( Id, Name). If there are 10 records in the Emp table and 5 records in the Dept table, how many rows will be displayed in the result of the following SQL query:

The query will result in 50 rows as a "cartesian product" or "cross join", which is the default whenever the 'where' clause is omitted.

16. What are all the different types of indexes?

There are three types of indexes -. Unique Index. This indexing does not allow the field to have duplicate values if the column is unique indexed. Unique index can be applied automatically when primary key is defined. Clustered Index. This type of index reorders the physical order of the table and search based on the key values. Each table can have only one clustered index. NonClustered Index. NonClustered Index does not alter the physical order of the table and maintains logical order of data. Each table can have 999 nonclustered indexes.

what does "UNION" do? what's the difference between "UNION" and "UNION ALL"?

UNION merges the contents of two structurally-compatible tables into a single combined table. The difference between UNION and UNION ALL is that UNION will omit duplicate records whereas UNION ALL will include duplicate records. It is important to note that the performance of UNION ALL will typically be better than UNION, since UNION requires the server to do the additional work of removing any duplicates. So, in cases where there is certain that there will not be any duplicates, or where having duplicates is not a problem, use of UNION ALL would be recommended for performance reasons.

What is Union, minus and Interact commands?

UNION operator is used to combine the results of two tables, and it eliminates duplicate rows from the tables. MINUS operator is used to return rows from the first query but not from the second query. Matching records of first and second query and other rows from the first query will be displayed as a result set. INTERSECT operator is used to return rows returned by both the queries.

What is the difference between the WHERE and HAVING clauses?

When GROUP BY is not used, the WHERE and HAVING clauses are essentially equivalent. However, when GROUP BY is used: The WHERE clause is used to filter records from a result. The filtering occurs before any groupings are made. The HAVING clause is used to filter values from a group (i.e., to check conditions after aggregation into groups has been performed).

What is the difference between char and varchar2?

When stored in a database, varchar2 uses only the allocated space. E.g. if you have a varchar2(1999) and put 50 bytes in the table, it will use 52 bytes. But when stored in a database, char always uses the maximum length and is blank-padded. E.g. if you have char(1999) and put 50 bytes in the table, it will consume 2000 bytes.

Write a SQL query using UNION ALL (not UNION) that uses the WHERE clause to eliminate duplicates. Why might you want to do this?

You can avoid duplicates using UNION ALL and still run much faster than UNION DISTINCT (which is actually same as UNION) by running a query like this: SELECT * FROM mytable WHERE a=X UNION ALL SELECT * FROM mytable WHERE b=Y AND a!=X The key is the AND a!=X part. This gives you the benefits of the UNION (a.k.a., UNION DISTINCT) command, while avoiding much of its performance hit.

How to find a duplicate record? duplicate records with one field duplicate records with more than one field

duplicate records with one field SELECT name, COUNT(email) FROM users GROUP BY email HAVING COUNT(email) > 1 duplicate records with more than one field SELECT name, email, COUNT(*) FROM users GROUP BY name, email HAVING COUNT(*) > 1