MySQL Study之--MySQL 表连接_MySQL

程序员文章站 2022-05-05 17:25:55

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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！！！

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;

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;

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