SQL中的INNER JOIN和JOIN有什么区别

2025-02-13 19:54:59
推荐回答(1个)
回答1:

没区别,inner join 是内连接 join默认就是inner join。

Table A
aid adate
1 a1
2 a2
3 a3

TableB

bid bdate
1 b1
2 b2
4 b4
两个表a,b相连接,要取出id相同的字段
select * from a inner join b on a.aid = b.bid这是仅取出匹配的数据.
此时的取出的是:
1 a1 b1
2 a2 b2

那么left join 指:
select * from a left join b on a.aid = b.bid
首先取出a表中所有数据,然后再加上与a,b匹配的的数据
此时的取出的是:
1 a1 b1
2 a2 b2
3 a3 空字符

同样的也有right join
指的是首先取出b表中所有数据,然后再加上与a,b匹配的的数据
此时的取出的是:
1 a1 b1
2 a2 b2
4 空字符 b4

LEFT JOIN 或 LEFT OUTER JOIN。
左向外联接的结果集包括 LEFT OUTER 子句中指定的左表的所有行,而不仅仅是联接列所匹配的行。如果左表的某行在右表中没有匹配行,则在相关联的结果集行中右表的所有选择列表列均为空值

二. left join/right join/inner join操作演示

表A记录如下:
aID aNum
1 a20050111
2 a20050112
3 a20050113
4 a20050114
5 a20050115

表B记录如下:
bID bName
1 2006032401
2 2006032402
3 2006032403
4 2006032404
8 2006032408

实验如下:
1. left join
sql语句如下:
SELECT * FROM A
LEFT JOIN B
ON A.aID = B.bID
结果如下:
aID aNum bID bName
1 a20050111 1 2006032401
2 a20050112 2 2006032402
3 a20050113 3 2006032403
4 a20050114 4 2006032404
5 a20050115 NULL NULL
(所影响的行数为 5 行)

结果说明:
left join是以A表的记录为基础的,A可以看成左表,B可以看成右表,left join是以左表为准的.
换句话说,左表(A)的记录将会全部表示出来,而右表(B)只会显示符合搜索条件的记录(例子中为: A.aID = B.bID).
B表记录不足的地方均为NULL.

2. right join
sql语句如下:
SELECT * FROM A
RIGHT JOIN B
ON A.aID = B.bID
结果如下:
aID aNum bID bName
1 a20050111 1 2006032401
2 a20050112 2 2006032402
3 a20050113 3 2006032403
4 a20050114 4 2006032404
NULL NULL 8 2006032408
(所影响的行数为 5 行)

结果说明:
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(B)为基础的,A表不足的地方用NULL填充.

3.inner join
sql语句如下:
SELECT * FROM A
INNERJOIN B
ON A.aID = B.bID
结果如下:
aID aNum bID bName
1 a20050111 1 2006032401
2 a20050112 2 2006032402
3 a20050113 3 2006032403
4 a20050114 4 2006032404

结果说明:
很明显,这里只显示出了 A.aID = B.bID的记录.这说明inner join并不以谁为基础,它只显示符合条件的记录.

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