wwq518的博客

SQL

select 1 from table_name

SQL

join

The reason is that the EXISTS operator works based on the “at least found” principle. It returns true and stops scanning table once at least one matching row found.On the other hands, when the IN opera

join-vs-sub-query

SQL 基础

1. 取出每组中的最后一条记录： SELECT m1.* FROM messages m1 LEFT JOIN messages m2 ON (m1.name = m2.name AND m1.id < m2.id) WHERE m2.id IS NULL; 2. 在数据库中，union和union all关键字都是将两个结果集合并为一个，但这两者从使用和效率上来说都有所不同。 ...

– 查出 A 表中，col 不等于 A 的数据，col 中可能有 null； SELECT * FROM A WHERE col != ‘A’ SELECT * FROM A WHERE col != ‘A’ OR col IS NULL – MYSQL中，NULL值与任何值比较永不为真 SELECT NULL != ‘A’ – NULL表示的是“a missing unknown value”，...