关于union和子查询过程中的给表起别名（个人笔记） |
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一段语句不能运行： select status,count(*) from (select invoice_id,vendor_id,invoice_total,'1st' as status from invoices where invoice_total > 1000 union select invoice_id,vendor_id,invoice_total,'2nd' as status from invoices where invoice_total between 500 and 1000 union select invoice_id,vendor_id,invoice_total,'3rd' as status from invoices where invoice_total 1000 union select invoice_id,vendor_id,invoice_total,'2nd' as status from invoices where invoice_total between 500 and 1000 union select invoice_id,vendor_id,invoice_total,'3rd' as status from invoices where invoice_total 1000 union select invoice_id,vendor_id,invoice_total,'2nd' as status from invoices where invoice_total between 500 and 1000 union select invoice_id,vendor_id,invoice_total,'3rd' as status from invoices where invoice_total (select max(sal) from emp where deptno = 30) 子查询无法使用的情况：1.group by后面不能使用子查询 2.单行子查询使用单行操作符，多行子查询使用多行操作符 单行：运算符等， 多行：in（等于列表中的任意一个） any（和子查询返回的任意一个值比较） all（和子查询返回的所有值比较） 3.子查询中的空值问题 是：where empno in (select mgr from emp) 否：where empno not in (select mgr from emp where mgr is not null) 如果用not in查询，要注意空值，否则可能返回0行 拓展子查询可以和多表查询相互代替 TOP-N问题中的伪列：rownum（只能在Oracle中使用） |
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