mysql新手入门随笔4
40、子查询:出现在其他SQL语句里的SELECT语句
例如:SELECT sname,mark FROM student WHERE mark = (SELECT max(mark) FROM student);
SELECT sname,mark FROM student WHERE mark in(SELECT max(mark) FROM student);
等效于:SELECT sname,mark FROM student WHERE mark =any(SELECT max(mark) FROM student);
in等效于=any
not in等效于!=all或<>all
41、多表连接查询
内连接:INNER JOIN,JOIN,CROSS JOIN,NATURAL JOIN
意思:两张表的交集,仅显示符合连接条件的记录,不符合的不显示
用法:SELECT a.*,b.* FROM pa a INNER JOIN pb bON a.id = b.id;
左(外)连接:LEFT (OUTER) JOIN
意思:显示左表全部和左右表中符合连接条件的记录
用法:SELECT a.*,b.* FROM pa a LEFT (OUTER)JOIN pb bON a.id = b.id;
右(外)连接:RIGHT (OUTER) JOIN
意思:显示右表全部和左右表中符合连接条件的记录
用法:SELECT a.*,b.* FROM pa a RIGHT (OUTER)JOIN pb bON a.id = b.id;
MySQL里的全连接
SELECT a.*,b.* FROM pa a LEFT (OUTER)JOIN pb bON a.id = b.id
UNION
SELECT a.*,b.* FROM pa a RIGHT (OUTER)JOIN pb bON a.id = b.id;
UNION:会对结果去重
UNION ALL:不会对结果去重