MySQL Study之--MySQL 表连接
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 condition
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FULL join.
案例分析：
1、案例环境
mysql> select * from emp;
+-------+--------+-----------+------+------------+------+------+--------+
| empno | ENAME | JOB | MGR | HIRE | SAL | COMM | deptno |
+-------+--------+-----------+------+------------+------+------+--------+
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800 | NULL | 20 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600 | 300 | 30 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250 | 500 | 30 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975 | NULL | 20 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250 | 1400 | 30 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850 | NULL | 30 |
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450 | NULL | 10 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-07-13 | 3000 | NULL | 20 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000 | NULL | 10 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500 | 0 | 30 |
| 7876 | ADAMS | CLERK | 7788 | 1987-06-13 | 1100 | NULL | 20 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950 | NULL | 30 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000 | NULL | 20 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300 | NULL | 10 |
+-------+--------+-----------+------+------------+------+------+--------+
14 rows in set (0.00 sec)
mysql> select * from dept;
+--------+------------+---------+
| deptNO | DNAME | LOC |
+--------+------------+---------+
| 10 | ACCOUNTING | NEWYORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+---------+
4 rows in set (0.00 sec)
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。
mysql> select e.empno,e.ename,e.sal,e.deptno,d.dname
-> from emp e
-> inner join dept d
-> where e.deptno=d.deptno;
+-------+--------+------+--------+------------+
| empno | ename | sal | deptno | dname |
+-------+--------+------+--------+------------+
| 7782 | CLARK | 2450 | 10 | ACCOUNTING |
| 7839 | KING | 5000 | 10 | ACCOUNTING |
| 7934 | MILLER | 1300 | 10 | ACCOUNTING |
| 7369 | SMITH | 800 | 20 | RESEARCH |
| 7566 | JONES | 2975 | 20 | RESEARCH |
| 7788 | SCOTT | 3000 | 20 | RESEARCH |
| 7876 | ADAMS | 1100 | 20 | RESEARCH |
| 7902 | FORD | 3000 | 20 | RESEARCH |
| 7499 | ALLEN | 1600 | 30 | SALES |
| 7521 | WARD | 1250 | 30 | SALES |
| 7654 | MARTIN | 1250 | 30 | SALES |
| 7698 | BLAKE | 2850 | 30 | SALES |
| 7844 | TURNER | 1500 | 30 | SALES |
| 7900 | JAMES | 950 | 30 | SALES |
+-------+--------+------+--------+------------+
14 rows in set (0.00 sec)
mysql> select e.empno,e.ename,e.sal,e.deptno,d.dname
-> from emp e
-> inner join dept d
-> on e.deptno=d.deptno;
+-------+--------+------+--------+------------+
| empno | ename | sal | deptno | dname |
+-------+--------+------+--------+------------+
| 7782 | CLARK | 2450 | 10 | ACCOUNTING |
| 7839 | KING | 5000 | 10 | ACCOUNTING |
| 7934 | MILLER | 1300 | 10 | ACCOUNTING |
| 7369 | SMITH | 800 | 20 | RESEARCH |
| 7566 | JONES | 2975 | 20 | RESEARCH |
| 7788 | SCOTT | 3000 | 20 | RESEARCH |
| 7876 | ADAMS | 1100 | 20 | RESEARCH |
| 7902 | FORD | 3000 | 20 | RESEARCH |
| 7499 | ALLEN | 1600 | 30 | SALES |
| 7521 | WARD | 1250 | 30 | SALES |
| 7654 | MARTIN | 1250 | 30 | SALES |
| 7698 | BLAKE | 2850 | 30 | SALES |
| 7844 | TURNER | 1500 | 30 | SALES |
| 7900 | JAMES | 950 | 30 | SALES |
+-------+--------+------+--------+------------+
14 rows in set (0.00 sec)
隐式内连接：
mysql> select e.empno,e.ename,e.sal,e.deptno,d.dname
-> from emp e,dept d
-> where e.deptno=d.deptno;
+-------+--------+------+--------+------------+
| empno | ename | sal | deptno | dname |
+-------+--------+------+--------+------------+
| 7782 | CLARK | 2450 | 10 | ACCOUNTING |
| 7839 | KING | 5000 | 10 | ACCOUNTING |
| 7934 | MILLER | 1300 | 10 | ACCOUNTING |
| 7369 | SMITH | 800 | 20 | RESEARCH |
| 7566 | JONES | 2975 | 20 | RESEARCH |
| 7788 | SCOTT | 3000 | 20 | RESEARCH |
| 7876 | ADAMS | 1100 | 20 | RESEARCH |
| 7902 | FORD | 3000 | 20 | RESEARCH |
| 7499 | ALLEN | 1600 | 30 | SALES |
| 7521 | WARD | 1250 | 30 | SALES |
| 7654 | MARTIN | 1250 | 30 | SALES |
| 7698 | BLAKE | 2850 | 30 | SALES |
| 7844 | TURNER | 1500 | 30 | SALES |
| 7900 | JAMES | 950 | 30 | SALES |
+-------+--------+------+--------+------------+
14 rows in set (0.00 sec)
left join：取得左表（table1）完全记录，即是右表（table2）并无对应匹配记录。
mysql> update emp set deptno=null where empno=7788;
Query OK, 1 row affected (0.07 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from emp where empno=7788;
+-------+-------+---------+------+------------+------+------+--------+
| empno | ENAME | JOB | MGR | HIRE | SAL | COMM | deptno |
+-------+-------+---------+------+------------+------+------+--------+
| 7788 | SCOTT | ANALYST | 7566 | 1987-07-13 | 3000 | NULL | NULL |
+-------+-------+---------+------+------------+------+------+--------+
1 row in set (0.00 sec)
采用等值连接：
mysql> select e.empno,e.ename,e.sal,e.deptno,d.dname
-> from emp e
-> inner join dept d on e.deptno=d.deptno;
+-------+--------+------+--------+------------+
| empno | ename | sal | deptno | dname |
+-------+--------+------+--------+------------+
| 7782 | CLARK | 2450 | 10 | ACCOUNTING |
| 7839 | KING | 5000 | 10 | ACCOUNTING |
| 7934 | MILLER | 1300 | 10 | ACCOUNTING |
| 7369 | SMITH | 800 | 20 | RESEARCH |
| 7566 | JONES | 2975 | 20 | RESEARCH |
| 7876 | ADAMS | 1100 | 20 | RESEARCH |
| 7902 | FORD | 3000 | 20 | RESEARCH |
| 7499 | ALLEN | 1600 | 30 | SALES |
| 7521 | WARD | 1250 | 30 | SALES |
| 7654 | MARTIN | 1250 | 30 | SALES |
| 7698 | BLAKE | 2850 | 30 | SALES |
| 7844 | TURNER | 1500 | 30 | SALES |
| 7900 | JAMES | 950 | 30 | SALES |
+-------+--------+------+--------+------------+
13 rows in set (0.00 sec)
-----对于等值连接，只能看到条件匹配的记录！
mysql> select e.empno,e.ename,e.sal,e.deptno,d.dname
-> from emp e
-> left join dept d on e.deptno=d.deptno;
+-------+--------+------+--------+------------+
| empno | ename | sal | deptno | dname |
+-------+--------+------+--------+------------+
| 7782 | CLARK | 2450 | 10 | ACCOUNTING |
| 7839 | KING | 5000 | 10 | ACCOUNTING |
| 7934 | MILLER | 1300 | 10 | ACCOUNTING |
| 7369 | SMITH | 800 | 20 | RESEARCH |
| 7566 | JONES | 2975 | 20 | RESEARCH |
| 7876 | ADAMS | 1100 | 20 | RESEARCH |
| 7902 | FORD | 3000 | 20 | RESEARCH |
| 7499 | ALLEN | 1600 | 30 | SALES |
| 7521 | WARD | 1250 | 30 | SALES |
| 7654 | MARTIN | 1250 | 30 | SALES |
| 7698 | BLAKE | 2850 | 30 | SALES |
| 7844 | TURNER | 1500 | 30 | SALES |
| 7900 | JAMES | 950 | 30 | SALES |
| 7788 | SCOTT | 3000 | NULL | NULL |
+-------+--------+------+--------+------------+
14 rows in set (0.00 sec)
-----通过left join可以查看到emp表中不符合条件的记录！
right join：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。
mysql> select e.empno,e.ename,e.sal,d.deptno,d.dname
-> from emp e
-> right join dept d on e.deptno=d.deptno;
+-------+--------+------+--------+------------+
| empno | ename | sal | deptno | dname |
+-------+--------+------+--------+------------+
| 7782 | CLARK | 2450 | 10 | ACCOUNTING |
| 7839 | KING | 5000 | 10 | ACCOUNTING |
| 7934 | MILLER | 1300 | 10 | ACCOUNTING |
| 7369 | SMITH | 800 | 20 | RESEARCH |
| 7566 | JONES | 2975 | 20 | RESEARCH |
| 7876 | ADAMS | 1100 | 20 | RESEARCH |
| 7902 | FORD | 3000 | 20 | RESEARCH |
| 7499 | ALLEN | 1600 | 30 | SALES |
| 7521 | WARD | 1250 | 30 | SALES |
| 7654 | MARTIN | 1250 | 30 | SALES |
| 7698 | BLAKE | 2850 | 30 | SALES |
| 7844 | TURNER | 1500 | 30 | SALES |
| 7900 | JAMES | 950 | 30 | SALES |
| NULL | NULL | NULL | 40 | OPERATIONS |
+-------+--------+------+--------+------------+
14 rows in set (0.00 sec)
------查询到dept表中，不符合条件的记录！！！
以下内容转自：http://www.jb51.net/article/50427.htm
浅析Mysql Join语法以及性能优化
在讲MySQL的Join语法前还是先回顾一下联结的语法，呵呵，其实连我自己都忘得差不多了，那就大家一起温习吧，这里我有个比较简便的记忆方法，内外联结的区别是内联结将去除所有不符合条件的记录，而外联结则保留其中部分。外左联结与外右联结的区别在于如果用A左联结B则A中所有记录都会保留在结果中，此时B中只有符合联结条件的记录，而右联结相反，这样也就不会混淆
了。
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FULL 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 |
+----+-----------+------+------+
| 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
#在执行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。
七.性能优化
1.显示(explicit) inner join VS 隐式(implicit) inner join
如：
select * from
table a inner join table b
on a.id = b.id;
VS
select a.*, b.*
from table a, table b
where a.id = b.id;
我在数据库中比较(10w数据)得之，它们用时几乎相同，第一个是显示的inner join，后一个是隐式的inner join。
2.left join/right join VS inner join
尽量用inner join.避免 LEFT JOIN 和 NULL.
在使用left join（或right join）时，应该清楚的知道以下几点：
(1). on与 where的执行顺序
ON 条件（“A LEFT JOIN B ON 条件表达式”中的ON）用来决定如何从 B 表中检索数据行。如果 B 表中没有任何一行数据匹配 ON 的条件,将会额外生成一行所有列为 NULL 的数据,在匹配阶段 WHERE 子句的条件都不会被使用。仅在匹配阶段完成以后，WHERE 子句条件才会被使用。它将从匹配阶段产生的数据中检索过滤。
所以我们要注意：在使用Left (right) join的时候，一定要在先给出尽可能多的匹配满足条件，减少Where的执行。如：
PASS
select * from A
inner join B on B.name = A.name
left join C on C.name = B.name
left join D on D.id = C.id
where C.status>1 and D.status=1;
select * from A
inner join B on B.name = A.name
left join C on C.name = B.name and C.status>1
left join D on D.id = C.id and D.status=1
从上面例子可以看出，尽可能满足ON的条件，而少用Where的条件。从执行性能来看第二个显然更加省时。
(2).注意ON 子句和 WHERE 子句的不同
如作者举了一个列子：
mysql> SELECT * FROM product LEFT JOIN product_details
ON (product.id = product_details.id)
AND product_details.id=2;
+----+--------+------+--------+-------+
| id | amount | id | weight | exist |
+----+--------+------+--------+-------+
| 1 | 100 | NULL | NULL | NULL |
| 2 | 200 | 2 | 22 | 0 |
| 3 | 300 | NULL | NULL | NULL |
| 4 | 400 | NULL | NULL | NULL |
+----+--------+------+--------+-------+
4 rows in set (0.00 sec)
mysql> SELECT * FROM product LEFT JOIN product_details
ON (product.id = product_details.id)
WHERE product_details.id=2;
+----+--------+----+--------+-------+
| id | amount | id | weight | exist |
+----+--------+----+--------+-------+
| 2 | 200 | 2 | 22 | 0 |
+----+--------+----+--------+-------+
1 row in set (0.01 sec)
从上可知，第一条查询使用 ON 条件决定了从 LEFT JOIN的 product_details表中检索符合的所有数据行。第二条查询做了简单的LEFT JOIN，然后使用 WHERE 子句从 LEFT JOIN的数据中过滤掉不符合条件的数据行。
(3).尽量避免子查询，而用join
往往性能这玩意儿，更多时候体现在数据量比较大的时候，此时，我们应该避免复杂的子查询。如下：
insert into t1(a1) select b1 from t2 where not exists(select 1 from t1 where t1.id = t2.r_id);
insert into t1(a1)
select b1 from t2
left join (select distinct t1.id from t1 ) t1 on t1.id = t2.r_id
where t1.id is null;