子查询要点-SQL语言基础

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文件名称：子查询要点-SQL语言基础
文件大小：5.26MB
文件格式：PPT
更新时间：2021-04-25 19:38:21
SQL 基础 子查询要点 * 子查询要包括在括号里面. 子查询要放在比较操作符的右边. 子查询中不要包含ORDER BY 子句。 对于单行的子查询使用单行的比较操作符 对于多行的子查询要使用多行的比较操作符. * Guidelines for Using Subqueries A subquery must be enclosed in parentheses. A subquery must appear on the right side of the comparison operator. Subqueries cannot contain an ORDER BY clause. You can have only one ORDER BY clause for a SELECT statement, and if specified it must be the last clause in the main SELECT statement. Two classes of comparison operators are used in subqueries: single-row operators and multiple-row operators. Instructor Note A subquery can execute multiple times in correlated subqueries, which are not included in this course. Students may ask how many subqueries can be written. The Oracle Server imposes no limit on the number of subqueries. The limit is related to the buffer size that the query uses.