mysql left join、right join、inner join用法分析
阿新 • • 發佈：2022-05-03
四種聯接
left join(左聯接) |
返回包括左表中的所有記錄和右表中聯結欄位相等的記錄 |
|---|---|
right join(右聯接) |
返回包括右表中的所有記錄和左表中聯結欄位相等的記錄 |
inner join(等值聯接) |
只返回兩個表中聯結欄位相等的行 |
cross join(交叉聯接) |
得到的結果是兩個表的乘積，即笛卡爾積 |
CREATE TABLE `product` (`id` int(10) unsigned not null auto_increment,`amount` int(10) unsigned default null,PRIMARY KEY (`id`)) ENGINE=innodb; CREATE TABLE `product_details` (`id` int(10) unsigned not null,`weight` int(10) unsigned default null,`exist` int(10) unsigned default null,PRIMARY KEY (`id`)) ENGINE=innodb;
插入資料
INSERT INTO product (id,amount) VALUES (1,100),(2,200),(3,300),(4,400);
INSERT INTO product_details (id,weight,exist) VALUES (2,22,0),(4,44,1),(5,55,0),(6,66,1);mysql> SELECT * FROM product; +----+--------+ | id | amount | +----+--------+ | 1 | 100 | | 2 | 200 | | 3 | 300 | | 4 | 400 | +----+--------+ 4 rows in set (0.00 sec) mysql> SELECT * FROM product_details; +----+--------+-------+ | id | weight | exist | +----+--------+-------+ | 2 | 22 | 0 | | 4 | 44 | 1 | | 5 | 55 | 0 | | 6 | 66 | 1 | +----+--------+-------+ 4 rows in set (0.00 sec)
inner join(等值聯接)
mysql> select * from product a inner join product_details b on a.id=b.id; +----+--------+----+--------+-------+ | id | amount | id | weight | exist | +----+--------+----+--------+-------+ | 2 | 200 | 2 | 22 | 0 | | 4 | 400 | 4 | 44 | 1 | +----+--------+----+--------+-------+ 2 rows in set (0.00 sec)
left join（左聯接）
mysql> select * from product a left join product_details b on a.id=b.id;
+----+--------+------+--------+-------+
| id | amount | id | weight | exist |
+----+--------+------+--------+-------+
| 1 | 100 | NULL | NULL | NULL |
| 2 | 200 | 2 | 22 | 0 |
| 3 | 300 | NULL | NULL | NULL |
| 4 | 400 | 4 | 44 | 1 |
+----+--------+------+--------+-------+
mysql> select * from product a left join product_details b on a.id=b.id and b.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 |
+----+--------+------+--------+-------+
mysql> select * from product a left join product_details b on a.id=b.id where b.id=2;
+----+--------+----+--------+-------+
| id | amount | id | weight | exist |
+----+--------+----+--------+-------+
| 2 | 200 | 2 | 22 | 0 |
+----+--------+----+--------+-------+
mysql> select * from product a left join product_details b on a.id=b.id where a.id=3;
+----+--------+------+--------+-------+
| id | amount | id | weight | exist|
+----+--------+------+--------+-------+
| 3 | 300 | NULL | NULL | NULL |
+----+--------+------+--------+-------+
mysql> select * from product a left join product_details b on a.id=b.id and a.id=3;
+----+--------+------+--------+-------+
| id | amount | id | weight | exist |
+----+--------+------+--------+-------+
| 1 | 100 | NULL | NULL | NULL |
| 2 | 200 | NULL | NULL | NULL |
| 3 | 300 | NULL | NULL | NULL |
| 4 | 400 | NULL | NULL | NULL |
+----+--------+------+--------+-------+
mysql> SELECT * FROM product a LEFT JOIN product_details b ON a.id=b.id AND b.weight!=44 AND b.exist=0 WHERE b.id IS NULL;
+----+--------+------+--------+-------+
| id | amount | id | weight | exist |
+----+--------+------+--------+-------+
| 1 | 100 | NULL | NULL | NULL |
| 3 | 300 | NULL | NULL | NULL |
| 4 | 400 | NULL | NULL | NULL |
+----+--------+------+--------+-------+
mysql> SELECT * FROM product a LEFT JOIN product_details b ON a.id=b.id AND b.weight!=44 AND b.exist=0 WHERE b.id IS not NULL;
+----+--------+------+--------+-------+
| id | amount | id | weight | exist |
+----+--------+------+--------+-------+
| 2 | 200 | 2 | 22 | 0 |
+----+--------+------+--------+-------+right join跟left join相反，不多做解釋，MySQL本身不支援所說的full join（全連線），但可以通過union來實現。
mysql> select * from product a left join product_details b on a.id=b.id;
+----+--------+------+--------+-------+
| id | amount | id | weight | exist |
+----+--------+------+--------+-------+
| 1 | 100 | NULL | NULL | NULL |
| 2 | 200 | 2 | 22 | 0 |
| 3 | 300 | NULL | NULL | NULL |
| 4 | 400 | 4 | 44 | 1 |
+----+--------+------+--------+-------+
mysql> select * from product a left join product_details b on a.id=b.id and b.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 |
+----+--------+------+--------+-------+
mysql> select * from product a left join product_details b on a.id=b.id where b.id=2;
+----+--------+----+--------+-------+
| id | amount | id | weight | exist |
+----+--------+----+--------+-------+
| 2 | 200 | 2 | 22 | 0 |
+----+--------+----+--------+-------+
mysql> select * from product a left join product_details b on a.id=b.id where a.id=3;
+----+--------+------+--------+-------+
| id | amount | id | weight | exist|
+----+--------+------+--------+-------+
| 3 | 300 | NULL | NULL | NULL |
+----+--------+------+--------+-------+
mysql> select * from product a left join product_details b on a.id=b.id and a.id=3;
+----+--------+------+--------+-------+
| id | amount | id | weight | exist |
+----+--------+------+--------+-------+
| 1 | 100 | NULL | NULL | NULL |
| 2 | 200 | NULL | NULL | NULL |
| 3 | 300 | NULL | NULL | NULL |
| 4 | 400 | NULL | NULL | NULL |
+----+--------+------+--------+-------+
mysql> SELECT * FROM product a LEFT JOIN product_details b ON a.id=b.id AND b.weight!=44 AND b.exist=0 WHERE b.id IS NULL;
+----+--------+------+--------+-------+
| id | amount | id | weight | exist |
+----+--------+------+--------+-------+
| 1 | 100 | NULL | NULL | NULL |
| 3 | 300 | NULL | NULL | NULL |
| 4 | 400 | NULL | NULL | NULL |
+----+--------+------+--------+-------+
mysql> SELECT * FROM product a LEFT JOIN product_details b ON a.id=b.id AND b.weight!=44 AND b.exist=0 WHERE b.id IS not NULL;
+----+--------+------+--------+-------+
| id | amount | id | weight | exist |
+----+--------+------+--------+-------+
| 2 | 200 | 2 | 22 | 0 |
+----+--------+------+--------+-------+Cross join（交叉聯接）
cross join：交叉聯接，得到的結果是兩個表的乘積，即笛卡爾積。
笛卡爾（Descartes）乘積又叫直積。
假設集合A={a,b}，集合B={0,1,2}，則兩個集合的笛卡爾積為{(a,0),(a,1),(a,2),(b,0),(b,1), (b,2)}。可以擴充套件到多個集合的情況。
類似的例子有，如果A表示某學校學生的集合，B表示該學校所有課程的集合，則A與B的笛卡爾積表示所有可能的選課情況。
mysql> select * from product a cross join product_details b;
+----+--------+----+--------+-------+
| id | amount | id | weight | exist |
+----+--------+----+--------+-------+
| 1 | 100 | 2 | 22 | 0 |
| 2 | 200 | 2 | 22 | 0 |
| 3 | 300 | 2 | 22 | 0 |
| 4 | 400 | 2 | 22 | 0 |
| 1 | 100 | 4 | 44 | 1 |
| 2 | 200 | 4 | 44 | 1 |
| 3 | 300 | 4 | 44 | 1 |
| 4 | 400 | 4 | 44 | 1 |
| 1 | 100 | 5 | 55 | 0 |
| 2 | 200 | 5 | 55 | 0 |
| 3 | 300 | 5 | 55 | 0 |
| 4 | 400 | 5 | 55 | 0 |
| 1 | 100 | 6 | 66 | 1 |
| 2 | 200 | 6 | 66 | 1 |
| 3 | 300 | 6 | 66 | 1 |
| 4 | 400 | 6 | 66 | 1 |
+----+--------+----+--------+-------+on與 where的執行順序
ON 條件（“A LEFT JOIN B ON 條件表示式”中的ON）用來決定如何從 B 表中檢索資料行。如果 B 表中沒有任何一行資料匹配 ON 的條件,將會額外生成一行所有列為 NULL 的資料,在匹配階段 WHERE 子句的條件都不會被使用。僅在匹配階段完成以後，WHERE 子句條件才會被使用。它將從匹配階段產生的資料中檢索過濾。
所以我們要注意：在使用Left (right) join的時候，一定要在先給出儘可能多的匹配滿足條件，減少Where的執行。
A Left join B On a.id=b.idAnd b.id=2;從B表中檢索符合的所有資料行，如果沒有匹配的全部為null A Left join B On a.id=b.idWhere b.id=2;先做left join 再過濾, WHERE 條件查詢發生在匹配階段之後 |
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