SQL小技巧

随机选择数据

SELECT * FROM table_name order by rand() limit 20;

INNER JOIN

两个表进行inner join时，对于相同的字段，不论是用左表还是右表，相交的结果都是一样的。

SELECT  a.column_name
FROM    (
        SELECT  column_name
        FROM    table_a
        ) a
JOIN (
        SELECT  column_name
        FROM    table_b
     ) b
ON      a.column_name = b.column_name
;

SELECT  b.column_name
FROM    (
        SELECT  column_name
        FROM    table_a
        ) a
JOIN (
        SELECT  column_name
        FROM    table_b
     ) b
ON      a.column_name = b.column_name
;

上面两个SQL的运行结果是相同的。