mysql多表关联leftjoin_Mysql多表表关联查询innerJoinleftjo。
。。
⼀.Join语法概述
join ⽤于多表中字段之间的联系,语法如下:... FROM table1 INNER|LEFT|RIGHT JOIN table2 ON conditiona
table1:左表;table2:右表。
JOIN 按照功能⼤致分为如下三类:
INNER JOIN(内连接,或等值连接):取得两个表中存在连接匹配关系的记录。
LEFT JOIN(左连接):取得左表(table1)完全记录,即是右表(table2)并⽆对应匹配记录。
RIGHT JOIN(右连接):与 LEFT JOIN 相反,取得右表(table2)完全记录,即是左表(table1)并⽆匹配对应记录。
注意:mysql不⽀持Full join,不过可以通过UNION 关键字来合并 LEFT JOIN 与 RIGHT JOIN来模拟FU
LL join.
redis命令图形界面接下来给出⼀个列⼦⽤于解释下⾯⼏种分类。如下两个表(A,B)mysql> select A.id,A.name,B.name from A,B where A.id=B.id;
+----+-----------+-------------+
| id | name | name |
+----+-----------+-------------+
| 1 | Pirate | Rutabaga |
| 2 | Monkey | Pirate |
| 3 | Ninja | Darth Vader |
| 4 | Spaghetti | Ninja |
+----+-----------+-------------+
4 rows in set (0.00 sec)
⼆.Inner join
内连接,也叫等值连接,inner join产⽣同时符合A和B的⼀组数据。mysql> select * from A inner join B on A.name = B.name;
+----+--------+----+--------+
| id | name | id | name |
+----+--------+----+--------+
| 1 | Pirate | 2 | Pirate |
| 3 | Ninja | 4 | Ninja |
+----+--------+----+--------+
三.Left join
mysql> select * from A left join B on A.name = B.name;
#或者:select * from A left outer join B on A.name = B.name;
+----+-----------+------+--------+
| id | name | id | name |
+----+-----------+------+--------+
| 1 | Pirate | 2 | Pirate |
| 2 | Monkey | NULL | NULL |
| 3 | Ninja | 4 | Ninja |
| 4 | Spaghetti | NULL | NULL |
+----+-----------+------+--------+
4 rows in set (0.00 sec)
left join,(或left outer join:在Mysql中两者等价,推荐使⽤left join.)左连接从左表(A)产⽣⼀套完整的记录,与匹配的记录(右表(B)) .如果没有匹配,右侧将包含null。
如果想只从左表(A)中产⽣⼀套记录,但不包含右表(B)的记录,可以通过设置where语句来执⾏,如下:mysql> select * from A left join B on A.name=B.name where A.id is null or B.id is null;
+----+-----------+------+------+
| id | name | id | name |
+----+-----------+------+------+
| 2 | Monkey | NULL | NULL |
| 4 | Spaghetti | NULL | NULL |
+----+-----------+------+------+
2 rows in set (0.00 sec)
同理,还可以模拟inner join. 如下:mysql> select * from A left join B on A.name=B.name where A.id is not null and B.id is not null;
+----+--------+------+--------+
| id | name | id | name |
+----+--------+------+--------+
| 1 | Pirate | 2 | Pirate |
| 3 | Ninja | 4 | Ninja |
+----+--------+------+--------+
2 rows in set (0.00 sec)
求差集:
sql表中添加数据根据上⾯的例⼦可以求差集,如下:SELECT * FROM A LEFT JOIN B ON A.name = B.name
WHERE B.id IS NULL
union
SELECT * FROM A right JOIN B ON A.name = B.name
WHERE A.id IS NULL;
# 结果
+------+-----------+------+-------------+
| id | name | id | name |
+------+-----------+------+-------------+
| 2 | Monkey | NULL | NULL |
| 4 | Spaghetti | NULL | NULL |
| NULL | NULL | 1 | Rutabaga |
| NULL | NULL | 3 | Darth Vader |
+------+-----------+------+-------------+
四.Right join
mysql语句多表查询mysql> select * from A right join B on A.name = B.name;
access安卓下载+------+--------+----+-------------+
| id | name | id | name |
+------+--------+----+-------------+
| NULL | NULL | 1 | Rutabaga |
| 1 | Pirate | 2 | Pirate |
| NULL | NULL | 3 | Darth Vader |
| 3 | Ninja | 4 | Ninja |
+------+--------+----+-------------+
4 rows in set (0.00 sec)
同left join。
五.Cross join
cross join:交叉连接,得到的结果是两个表的乘积,即笛卡尔积
笛卡尔(Descartes)乘积⼜叫直积。假设集合A={a,b},集合B={0,1,2},则两个集合的笛卡尔积为{(a,0),(a,1),(a,2),(b,0),(b,1), (b,2)}。可以扩展到多个集合的情况。类似的例⼦有,如果A表⽰某学校学⽣的集合,B表⽰该学校所有课程的集合,则A与B的笛卡尔积表⽰所有可能的选课情况。
mysql> select * from A cross join B;
+----+-----------+----+-------------+
| id | name | id | name |
+----+-----------+----+-------------+
| 1 | Pirate | 1 | Rutabaga |
| 2 | Monkey | 1 | Rutabaga |
| 3 | Ninja | 1 | Rutabaga |
| 4 | Spaghetti | 1 | Rutabaga |
| 1 | Pirate | 2 | Pirate |
| 2 | Monkey | 2 | Pirate |
| 3 | Ninja | 2 | Pirate |
| 4 | Spaghetti | 2 | Pirate |
| 1 | Pirate | 3 | Darth Vader |
| 2 | Monkey | 3 | Darth Vader |
| 3 | Ninja | 3 | Darth Vader |
| 4 | Spaghetti | 3 | Darth Vader |
| 1 | Pirate | 4 | Ninja |
| 2 | Monkey | 4 | Ninja |
| 3 | Ninja | 4 | Ninja |
| 4 | Spaghetti | 4 | Ninja |
+----+-----------+----+-------------+
16 rows in set (0.00 sec)
#再执⾏:mysql> select * from A inner join B; 试⼀试
#再执⾏mysql> select * from A cross join B on A.name = B.name; 试⼀试
实际上,在 MySQL 中(仅限于 MySQL) CROSS JOIN 与 INNER JOIN 的表现是⼀样的,在不指定 ON 条件得到的结果都是笛卡尔积,反之取得两个表完全匹配的结果。
INNER JOIN 与 CROSS JOIN 可以省略 INNER 或 CROSS 关键字,因此下⾯的 SQL 效果是⼀样的:... FROM table1 INNER JOIN table2
... FROM table1 CROSS JOIN table2
... FROM table1 JOIN table2
六.Full join
软件开发语言有哪几种mysql> select * from A left join B on B.name = A.name
余切函数图象做法-> union
-> select * from A right join B on B.name = A.name;
+------+-----------+------+-------------+
| id | name | id | name |
+------+-----------+------+-------------+
| 1 | Pirate | 2 | Pirate |
| 2 | Monkey | NULL | NULL |
| 3 | Ninja | 4 | Ninja |
| 4 | Spaghetti | NULL | NULL |
| NULL | NULL | 1 | Rutabaga |
| NULL | NULL | 3 | Darth Vader |
+------+-----------+------+-------------+
6 rows in set (0.00 sec)
全连接产⽣的所有记录(双⽅匹配记录)在表A和表B。如果没有匹配,则对⾯将包含null。
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