SQL frequently used commands and interview question and answers


Below are the continuous post for the frequently used commands and interview questions for SQL basic and intermediate level SQL Developers, Administrators, and Testers. These questions are about SELECT commands and some of the important and frequently  used commands.


21) What are the privileges that can be granted on a table by a user to others?
Insert, update, delete, select, references, index, execute, alter, all.

22) Which function is used to find the largest integer less than or equal to a specific value?
FLOOR.

23) Which is the subset of SQL commands used to manipulate Oracle Database structures, including tables?
Data Definition Language (DDL).

24) What is the use of DESC in SQL?
DESC has two purposes. It is used to describe a schema as well as to retrieve rows from table in descending order.
Explanation : The query SELECT * FROM EMP ORDER BY ENAME DESC will display the output sorted on ENAME in descending order.

25) What command is used to create a table by copying the structure of another table?
CREATE TABLE .. AS SELECTcommand
Explanation: To copy only the structure, the WHERE clause of the SELECT command should contain a FALSE statement as in the following.
CREATE TABLE NEWTABLE AS SELECT * FROM EXISTINGTABLE WHERE 1=2;
If the WHERE condition is true, then all the rows or rows satisfying the condition will be copied to the new table.

26) What is the output of the following query SELECT TRUNC(1234.5678,-2) FROM DUAL;?
1200.

27) What are the wildcards used for pattern matching.?
_ for single character substitution
% for multi-character substitution.

28) What's an SQL injection?
SQL Injection is when form data contains an SQL escape sequence and injects a new SQL query to be run.

29) What is difference between TRUNCATE & DELETE?
TRUNCATE commits after deleting entire table i.e., cannot be rolled back. Database triggers do not fire on TRUNCATE.
DELETE allows the filtered deletion. Deleted records can be rolled back or committed. Database triggers fire on DELETE.

30) What is a join? Explain the different types of joins?
Join is a query, which retrieves related columns or rows from multiple tables.
Self Join - Joining the table with itself.
Equi Join - Joining two tables by equating two common columns. Non-Equi Join - Joining two tables by equating two common columns.
Outer Join - Joining two tables in such a way that query can also retrieve rows that do not have corresponding join value in the other table.

31) What is the sub-query?
Sub-query is a query whose return values are used in filtering conditions of the main query.

32) What is correlated sub-query?
Correlated sub-query is a sub-query, which has reference to the main query.

33) Explain CONNECT BY PRIOR?
Retrieves rows in hierarchical order eg. select empno, ename from emp where.

34) Difference between SUBSTR and INSTR?
INSTR (String1, String2 (n, (m)),
INSTR returns the position of the m-th occurrence of the string 2 in string1. The search begins from nth position of string1.
SUBSTR (String1 n, m)
SUBSTR returns a character string of size m in string1, starting from n-th position of string1.

35) Explain UNION, MINUS, UNION ALL and INTERSECT?
INTERSECT - returns all distinct rows selected by both queries.
MINUS - returns all distinct rows selected by the first query but not by the second.
UNION - returns all distinct rows selected by either query
UNION ALL - returns all rows selected by either query,including all duplicates.

36) What is ROWID?
ROWID is a pseudo column attached to each row of a table. It is 18 characters long, blockno, rownumber are the components of ROWID.

37) What is the fastest way of accessing a row in a table?
Using ROWID. CONSTRAINTS

38) What is an integrity constraint?
Integrity constraint is a rule that restricts values to a column in a table.

39) What is referential integrity constraint?
Maintaining data integrity through a set of rules that restrict the values of one or more columns of the tables based on the values of primary key or unique key of the referenced table.




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