mysql---多表查询
很多应用场景下，数据查询会涉及多张表，即多表查询。这些表之间存在连结关系。
多表连结分为：内连结，外连结, 交叉连结。
针对内外连结对应不同的连结方法
- 内连结：(inner)join
- 外连结：left(outer) join, right(outer) join, union
- 交叉连结：cross join
其中，
1. join和inner join是等价的
2. left join和left outer join是等价的，right join和right outer join是等价的；
3. union本来不属于连结方法，用来组合其他连结方法实现新的连结方法。
下面通过实例对这些连结方法进行描述，首先建立两张表
表a:
+------+---------+
| c_id | country |
+------+---------+
| 1 | CHINA |
| 2 | UK |
| 3 | USA |
+------+---------+表b:
+------+--------+
| n_id | name |
+------+--------+
| 2 | Mike |
| 3 | Hanson |
| 4 | Jack |
+------+--------+1. 内连结
使用inner join或者join
SELECT a.*, b.*
FROM a JOIN b
ON a.c_id = b.n_id;
或者
SELECT a.*, b.*
FROM a INNER JOIN b
ON a.c_id = b.n_id;
结果：
+------+---------+------+--------+
| c_id | country | n_id | name |
+------+---------+------+--------+
| 2 | UK | 2 | Mike |
| 3 | USA | 3 | Hanson |
+------+---------+------+--------+求两个表的交集
2. 外连结
2.1 左外连结1
使用left outer join或者left join
SELECT a.*, b.*
FROM a LEFT JOIN b
ON a.c_id = b.n_id;
或者:
SELECT a.*, b.*
FROM a LEFT OUTER JOIN b
ON a.c_id = b.n_id;
结果:
+------+---------+------+--------+
| c_id | country | n_id | name |
+------+---------+------+--------+
| 2 | UK | 2 | Mike |
| 3 | USA | 3 | Hanson |
| 1 | CHINA | NULL | NULL |
+------+---------+------+--------+以a表为基准，b表中没有的直接补NULL
这样得到的结果是，a的所有数据和满足一定条件的b的部分数据
2.2 左外连结2
SELECT a.*, b.*
FROM a LEFT JOIN b
ON a.c_id = b.n_id
WHERE b.n_id IS NULL;
+------+---------+------+------+
| c_id | country | n_id | name |
+------+---------+------+------+
| 1 | CHINA | NULL | NULL |
+------+---------+------+------+2.3 右外连结1
SELECT a.*, b.*
FROM a RIGHT JOIN b
ON a.c_id = b.n_id
WHERE a.c_id IS NULL;
+------+---------+------+------+
| c_id | country | n_id | name |
+------+---------+------+------+
| NULL | NULL | 4 | Jack |
+------+---------+------+------+2.4 右外连结2
SELECT a.*, b.*
FROM a RIGHT JOIN b
ON a.c_id = b.n_id;
+------+---------+------+--------+
| c_id | country | n_id | name |
+------+---------+------+--------+
| 2 | UK | 2 | Mike |
| 3 | USA | 3 | Hanson |
| NULL | NULL | 4 | Jack |
+------+---------+------+--------+2.5 全连结1
mysql不支持这种连结，但可以使用left join union right join来实现
SELECT a.*, b.*
FROM a LEFT JOIN b
ON a.c_id = b.n_id
UNION
SELECT a.*, b.*
FROM a RIGHT JOIN b
ON a.c_id = b.n_id;
+------+---------+------+--------+
| c_id | country | n_id | name |
+------+---------+------+--------+
| 2 | UK | 2 | Mike |
| 3 | USA | 3 | Hanson |
| 1 | CHINA | NULL | NULL |
| NULL | NULL | 4 | Jack |
+------+---------+------+--------+2.6 全连结2
SELECT a.*, b.*
FROM a LEFT JOIN b
ON a.c_id = b.n_id
WHERE b.n_id IS NULL
UNION
SELECT a.*, b.*
FROM a RIGHT JOIN b
ON a.c_id = b.n_id
WHERE a.c_id IS NULL;
+------+---------+------+------+
| c_id | country | n_id | name |
+------+---------+------+------+
| 1 | CHINA | NULL | NULL |
| NULL | NULL | 4 | Jack |
+------+---------+------+------+3. 交叉连结
得到两个表的笛卡尔积
SELECT a.*
FROM a CROSS JOIN b;
+------+---------+
| c_id | country |
+------+---------+
| 1 | CHINA |
| 2 | UK |
| 3 | USA |
| 1 | CHINA |
| 2 | UK |
| 3 | USA |
| 1 | CHINA |
| 2 | UK |
| 3 | USA |
+------+---------+