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Sql Interview Questions and Answers



What command is used to create a table by copying the structure of another table?
CREATE TABLE .. AS SELECT command
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.

TRUNCATE TABLE EMP; DELETE FROM EMP; Will the outputs of the above two commands differ?
Both will result in deleting all the rows in the table EMP..  

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

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

What is the parameter substitution symbol used with INSERT INTO command?
&

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.

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.

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.

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

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

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

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.

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.

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.

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

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