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 b ON 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 b ON 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 b ON a.id = b.id;

 

MySQL里的全连接

SELECT a.*,b.* FROM pa a LEFT (OUTER) JOIN pb b ON a.id = b.id

UNION

SELECT a.*,b.* FROM pa a RIGHT (OUTER) JOIN pb b ON a.id = b.id;

 

UNION：会对结果去重

UNION ALL：不会对结果去重