MySQL에서 Join 사용 사례 공유

MySQL(5.1을 예로 들어)에서 테이블 조인 구문은 MySQL 공식 매뉴얼에서 찾을 수 있습니다: MySQL 공식 매뉴얼-JOIN

쿼리에서 조인 구문은 유사합니다

SELECT select_expr FROM table_references

table_references(참조 to tables) 의 정의는 다음과 같습니다(연결 표현식이라고도 볼 수 있음): (흐릿한 후광 ㅋㅋ)

table_references:
    table_reference [, table_reference] ...

table_reference:
    table_factor
  | join_table

table_factor:
    tbl_name [[AS] alias] [index_hint_list]
  | table_subquery [AS] alias
  | ( table_references )
  | { OJ table_reference LEFT OUTER JOIN table_reference
        ON conditional_expr }

join_table:
    table_reference [INNER | CROSS] JOIN table_factor [join_condition]
  | table_reference STRAIGHT_JOIN table_factor
  | table_reference STRAIGHT_JOIN table_factor ON conditional_expr
  | table_reference {LEFT|RIGHT} [OUTER] JOIN table_reference join_condition
  | table_reference NATURAL [{LEFT|RIGHT} [OUTER]] JOIN table_factor

join_condition:
    ON conditional_expr
  | USING (column_list)

index_hint_list:
    index_hint [, index_hint] ...

index_hint:
    USE {INDEX|KEY}
      [{FOR {JOIN|ORDER BY|GROUP BY}] ([index_list])
  | IGNORE {INDEX|KEY}
      [{FOR {JOIN|ORDER BY|GROUP BY}] (index_list)
  | FORCE {INDEX|KEY}
      [{FOR {JOIN|ORDER BY|GROUP BY}] (index_list)

index_list:
    index_name [, index_name] ...

그 중 table_factor가 기본 테이블 선택이고, Join_table은 일부를 기반으로 합니다. 테이블 확장.
이제 실험을 통해 테이블 ​​연결을 소개하겠습니다.
먼저 다음과 같은 테이블이 있다고 가정합니다.

phptable2id

3

author
2	zhang

3

wang

litable3author

4

year
zhang	2003

ma

2006

2011

Inner Join 内连接
将两个表中存在连接关系的字段，组成的记录集，叫做内连接。
内连接等价于

mysql> select table1.id as id,book,author from table1, table2 where table1.id=table2.id;
+------+------+--------+
| id   | book | author |
+------+------+--------+
|    2 | c++  | zhang  |
|    3 | php  | wang   |
+------+------+--------+
2 rows in set (0.00 sec)
mysql> select * from table1 inner join table2 using (id);
+------+------+--------+
| id   | book | author |
+------+------+--------+
|    2 | c++  | zhang  |
|    3 | php  | wang   |
+------+------+--------+
2 rows in set (0.00 sec)

可以看出，两者是等价的。没有Using子句的Inner Join相当于是求两个table的笛卡尔积。
Cross Join 交叉连接
在Mysql中，Cross Join可以用逗号表达式表示，例如(table1, table 2)。在Mysql中，Cross Join 和 Inner Join 是等价的，但是在标准SQL中，它们并不等价，Inner Join 用于带有on表达式的连接，反之用Cross Join。以下两个SQL语句是等价的。
Cross Join 指的是两个table的笛卡尔积。以下三句SQL是等价的。

mysql> select * from table1 inner join table2;
mysql> select * from table1 cross join table2;
mysql> select * from (table1, table2);
mysql> select * from table1 nature join table2;
结果集：
+------+------+------+--------+
| id   | book | id   | author |
+------+------+------+--------+
|    1 | java |    2 | zhang  |
|    2 | c++  |    2 | zhang  |
|    3 | php  |    2 | zhang  |
|    1 | java |    3 | wang   |
|    2 | c++  |    3 | wang   |
|    3 | php  |    3 | wang   |
|    1 | java |    4 | li     |
|    2 | c++  |    4 | li     |
|    3 | php  |    4 | li     |
+------+------+------+--------+

不难理解，下面两句SQL也是等价的。

mysql> select * from table1 left join (table2, table3) on (table2.id = table1.id and table2.author = table3.author);
mysql> select * from table1 left join (table2 cross join table3) on (table2.id = table1.id and table2.author = table3.author);
结果集：
+------+------+------+--------+--------+------+
| id   | book | id   | author | author | year |
+------+------+------+--------+--------+------+
|    1 | java | NULL | NULL   | NULL   | NULL |
|    2 | c++  |    2 | zhang  | zhang  | 2003 |
|    3 | php  | NULL | NULL   | NULL   | NULL |
+------+------+------+--------+--------+------+

Natural Join 自然连接
NATURAL [LEFT] JOIN：这个句子的作用相当于INNER JOIN，或者是在USING子句中包含了联结的表中所有公共字段的Left JOIN（左联结）。
也就是说：下面两个SQL是等价的。

mysql> select * from table1 natural join table2;
mysql> select * from table1 inner join table2 using (id);

结果集：
+------+------+--------+
| id   | book | author |
+------+------+--------+
|    2 | c++  | zhang  |
|    3 | php  | wang   |
+------+------+--------+

同时，下面两个SQL也是等价的。

mysql> select * from table1 natural left join table2;
mysql> select * from table1 left join table2 using(id);
结果集：
+------+------+--------+
| id   | book | author |
+------+------+--------+
|    1 | java | NULL   |
|    2 | c++  | zhang  |
|    3 | php  | wang   |
+------+------+--------+

Left Join 左外连接
左外连接A、B表的意思就是将表A中的全部记录和表B中字段连接形成的记录集，这里注意的是最后出来的记录集会包括表A的全部记录。
左连接表1，表二等价于右连接表二，表一。如下两个SQL是等价的：

mysql> select * from table1 left join table2 using (id);
mysql> select * from table2 right join table1 using (id);
结果集：
+------+------+--------+
| id   | book | author |
+------+------+--------+
|    1 | java | NULL   |
|    2 | c++  | zhang  |
|    3 | php  | wang   |
+------+------+--------+

Right Join 右外连接

右外连接和左外连接是类似的。为了方便数据库便于访问，推荐使用左外连接代替右外连接。

最后，讲一下Mysql表连接的一些注意事项。

1、两个表求差集的方法
如果求 左表 - 右表 的差集，使用类似下面的SQL：

SELECT left_tbl.* FROM left_tbl LEFT JOIN right_tbl ON left_tbl.id = right_tbl.id WHERE right_tbl.id IS NULL;
例如
mysql> select table1.* from table1 left join table2 using(id) where table2.id is null;
+------+------+
| id   | book |
+------+------+
|    1 | java |
+------+------+
1 row in set (0.00 sec)

2、Using子句
Using子句可以使用On子句重写。但是使用Select *　查询出的结果有差别。以下两句话是等价的：

mysql> select id, book, author from table1 join table2 using (id);
mysql> select table1.id, book, author from table1 join table2 on table1.id=table2.id;
结果集：
+------+------+--------+
| id   | book | author |
+------+------+--------+
|    2 | c++  | zhang  |
|    3 | php  | wang   |
+------+------+--------+

但是下面两个有些许不同，使用on时候，重复的部分会被输出两次。

mysql> select * from table1 join table2 using (id);
+------+------+--------+
| id   | book | author |
+------+------+--------+
|    2 | c++  | zhang  |
|    3 | php  | wang   |
+------+------+--------+
2 rows in set (0.00 sec)
mysql> select * from table1 join table2 on table1.id=table2.id;
+------+------+------+--------+
| id   | book | id   | author |
+------+------+------+--------+
|    2 | c++  |    2 | zhang  |
|    3 | php  |    3 | wang   |
+------+------+------+--------+
2 rows in set (0.00 sec)

3、Straight Join的使用
STRAIGHT_JOIN 和 JOIN相似，除了大部分情况下，在使用STRAIGHT_JOIN时候，先读右表后读左表。而在大部分情况下是先读左表的。STRAIGHT_JOIN仅用于少数情况下的表连接性能优化，比如右表记录数目明显少于左表。

4、Mysql表连接的运算顺序
在MySQL 5.1版本中，INNER JOIN, CROSS JOIN, LEFT JOIN, 和RIGHT JOIN 比逗号表达式具有更高的优先级。
因此SQL1被解析成SQL3，而不是SQL2。

SQL1 :　SELECT * FROM t1, t2 JOIN t3 ON (t1.i1 = t3.i3);
SQL2 :　SELECT * FROM (t1, t2) JOIN t3 ON (t1.i1 = t3.i3);
SQL3 :　SELECT * FROM t1, (t2 JOIN t3 ON (t1.i1 = t3.i3));

因此会报错，找不到i1列。因此以后在写这样的查询的时候，最好写明白，不要省略括号，这样能避免很多错误。

5、循环的自然连接
在MySQL 5.1版本中，SQL1等价于SQL3， 而在MySQL以前版本中，SQL1等价于SQL2。

SQL1 : SELECT ... FROM t1 NATURAL JOIN t2 NATURAL JOIN t3;
SQL2 : SELECT ... FROM t1, t2, t3 WHERE t1.b = t2.b AND t2.c = t3.c;
SQL3 : SELECT ... FROM t1, t2, t3 WHERE t1.b = t2.b AND t2.c = t3.c AND t1.a = t3.a;

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