CREATE TABLE testA aID int( 1 ) AUTO_INCREMENT PRIMARY KEY , aNum char( 20 ) ) CREATE TABLE testB( bID int( 1 ) NOT NULL AUTO_INCREMENT PRIMARY KEY , bName char( 20 ) ) 表testA记录如下: aID aNum 1 a11 2 a22 3 a33 4 a44 5 a55 表testB记录如下: bID bName 1 b11 2 b22 3 b33 4 b44 8 b88
1.left join(左联接)
sql语句如下: SELECT * FROM testA LEFT JOIN testB ON testA.aID = testB.bID 结果如下: aID aNum bID bName 1 a11 1 b11 2 a22 2 b22 3 a33 3 b33 4 a44 4 b44 5 a55 NULL NULL
left join是以A表的记录为基础的,testA可以看成左表,testB可以看成右表,左表(testA)的记录将会全部表示出来,而右表(testB)只会显示符合搜索条件的记录(例子中为: testA.aID = testB.bID). testB表记录不足的地方均为NULL.
2.right join(右联接)
sql语句如下: SELECT * FROM testA RIGHT JOING testB ON testA.aID = testB.bID 结果如下: aID aNum bID bName 1 a11 1 b11 2 a22 2 b22 3 a33 3 b33 4 a44 4 b44 NULL NULL 8 b88
仔细观察一下,就会发现,和left join的结果刚好相反,这次是以右表(testB)为基础的,testA表不足的地方用NULL填充.
3.inner join(相等联接或内联接)
sql语句如下: SELECT * FROM testA INNER JOIN testB ON testA.aID = testB.bID 等同于以下SQL句: SELECT * FROM testA,testB WHERE testA.aID = testB.bID 结果如下: aID aNum bID bName 1 a11 1 b11 2 a22 2 b22 3 a33 3 b33 4 a44 4 b44
很明显,这里只显示出了 testA.aID = testB.bID的记录.这说明inner join它只显示符合条件的记录.