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.
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 |
sql left join 多表连接+----+-----------+------+------+
| 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：
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> select * from A right join B on A.name = B.name;
+------+--------+----+-------------+
| 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
mysql> select * from A left join B on B.name = A.name
-> 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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