SQL 面试题与答案
问题 106. What is the sub-query?
Sub-query is a query whose return values are used in filtering conditions of the main query.
问题 107. What is correlated sub-query?
Correlated sub-query is a sub-query, which has reference to the main query.
问题 108. Explain CONNECT BY PRIOR?
Retrieves rows in hierarchical order eg.
select empno, ename from emp where.
问题 109. 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.
问题 110. 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.
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