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SQL Interview Questions with Answers - Set 5
Posted Date: 06 Feb 2008 Resource Type: Articles/Knowledge Sharing Category: General
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Posted By: Bala Member Level: Diamond
Rating:  Points: 4
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SQL Interview Questions with Answers - Set 5
1. 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.
2. What is the sub-query?
Sub-query is a query whose return values are used in filtering conditions of the main query.
3. What is correlated sub-query?
Correlated sub-query is a sub-query, which has reference to the main query.
4. Explain CONNECT BY PRIOR?
Retrieves rows in hierarchical order eg.
select empno, ename from emp where.
5. 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.
6. 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.
7. 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.
8. What is the fastest way of accessing a row in a table?
Using ROWID.
CONSTRAINTS
9. What is an integrity constraint?
Integrity constraint is a rule that restricts values to a column in a table.
10. 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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