mysql 关联表关系

7种组合方式：

SELECT * FROM mhbtest.dept a inner JOIN mhbtest.emp b on a.deptno = b.deptid

SELECT * FROM mhbtest.dept a LEFT JOIN mhbtest.emp b on a.deptno = b.deptid

SELECT * FROM mhbtest.dept a RIGHT JOIN mhbtest.emp b on a.deptno = b.deptid

SELECT * FROM mhbtest.dept a LEFT JOIN mhbtest.emp b on a.deptno = b.deptid WHERE b.id is NULL

SELECT * FROM mhbtest.dept a RIGHT JOIN mhbtest.emp b on a.deptno = b.deptid WHERE a.id is NULL

SELECT * FROM mhbtest.dept a LEFT JOIN mhbtest.emp b on a.deptno = b.deptid union
SELECT * FROM mhbtest.dept a RIGHT JOIN mhbtest.emp b on a.deptno = b.deptid

SELECT * FROM mhbtest.dept a LEFT JOIN mhbtest.emp b on a.deptno = b.deptid WHERE b.deptid is null UNION
SELECT * FROM mhbtest.dept a RIGHT JOIN mhbtest.emp b on a.deptno = b.deptid WHERE a.deptno is null