sql各种连接笛卡尔积,inner join,left join,right join,full join等测试与区别
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首先原数据库的数据有:
TEST_A表:和TEST_B表:
一、笛卡尔积:
select * from TEST_A a,TEST_B b where a.ID = b.ID;
二、inner join:
select * from TEST_A a inner join TEST_B b on a.ID = b.ID;
三、left join:
select * from TEST_A a left join TEST_B b on a.ID = b.ID;
四、right join:
select * from TEST_A a right join TEST_B b on a.ID = b.ID;
五、full join:
select * from TEST_A a full join TEST_B b on a.ID = b.ID;
总结:笛卡尔积和inner join结果相同,但是inner join效率快一点
left join:TEST_A表的ID为空时拼接TEST_B表的内容为空,right join则相反
full join:等于left join和right join的并集
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