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正确理解left join

發(fā)布時間：2023/12/20 42 豆豆

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通俗來講，left join就是以左表作為主表，結(jié)果返回左表的所有記錄，右表滿足條件記錄正常顯示，滿足條件記錄使用NULL做填充，一般業(yè)務(wù)中我們需要顯示左表全部記錄時才會使用left join。另外，某些情況下MySQL優(yōu)化器會將我們的left join改寫為join，什么情況下MySQL會做這樣的優(yōu)化？

1.1 測試數(shù)據(jù)

root@mysql 11:24: [db1]> show create table t1\G *************************** 1. row ***************************Table: t1 Create Table: CREATE TABLE `t1` (`uid` int(11) NOT NULL AUTO_INCREMENT,`uname` varchar(10) DEFAULT NULL,`is_delete` int(11) DEFAULT '0',PRIMARY KEY (`uid`),KEY `idx_uname` (`uname`) ) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8mb4 1 row in set (0.00 sec)root@mysql 11:24: [db1]> show create table t2\G *************************** 1. row ***************************Table: t2 Create Table: CREATE TABLE `t2` (`id` int(11) NOT NULL AUTO_INCREMENT,`uname` varchar(10) DEFAULT NULL,`uage` varchar(10) DEFAULT NULL,PRIMARY KEY (`id`),KEY `idx_uname` (`uname`) ) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8mb4 1 row in set (0.00 sec)root@mysql 11:24: [db1]> select * from t1; +-----+-------+-----------+ | uid | uname | is_delete | +-----+-------+-----------+ | 1 | aa | 0 | | 2 | bb | 0 | | 3 | cc | 0 | | 4 | dd | 0 | | 5 | ee | 0 | +-----+-------+-----------+ 5 rows in set (0.00 sec)root@mysql 11:24: [db1]> select * from t2; +----+-------+------+ | id | uname | uage | +----+-------+------+ | 1 | aa | 12 | | 2 | aa | 13 | | 5 | cc | 12 | +----+-------+------+ 3 rows in set (0.00 sec)

1.2 條件放在on和where之前的區(qū)別

1、從結(jié)果顯示來看

從結(jié)果顯示來看兩者是完全不同的：

1）當(dāng)把過濾條件寫在and上時，返回結(jié)果集中會顯示左表全部記錄，右表滿足條件的記錄正常顯示，不滿足條件的記錄顯示為NULL，是我們通俗理解上left join應(yīng)該顯示的結(jié)果；

2）而當(dāng)把過濾條件寫在where上時，雖然SQL中我們使用了left join去做表關(guān)聯(lián)，但是實際結(jié)果集中并不是我們想要的返回，結(jié)果只是返回了滿足條件的所有記錄。

root@mysql 11:24: [db1]> select * from t1 left join t2 on t1.uname=t2.uname and t2.uage>12; +-----+-------+-----------+------+-------+------+ | uid | uname | is_delete | id | uname | uage | +-----+-------+-----------+------+-------+------+ | 1 | aa | 0 | 2 | aa | 13 | | 2 | bb | 0 | NULL | NULL | NULL | | 3 | cc | 0 | NULL | NULL | NULL | | 4 | dd | 0 | NULL | NULL | NULL | | 5 | ee | 0 | NULL | NULL | NULL | +-----+-------+-----------+------+-------+------+ 5 rows in set (0.00 sec)root@mysql 11:25: [db1]> select * from t1 left join t2 on t1.uname=t2.uname where t2.uage>12; +-----+-------+-----------+------+-------+------+ | uid | uname | is_delete | id | uname | uage | +-----+-------+-----------+------+-------+------+ | 1 | aa | 0 | 2 | aa | 13 | +-----+-------+-----------+------+-------+------+ 1 row in set (0.00 sec)

2、從執(zhí)行計劃來看

1）當(dāng)把過濾條件寫在and上時，執(zhí)行計劃沒有做過多的改寫，左表t1作為驅(qū)動表與t2進(jìn)行關(guān)聯(lián)查詢；

2）當(dāng)把過濾條件寫在where上時，我們發(fā)現(xiàn)MySQL對原SQL進(jìn)行了改寫，最重要的一點是將left join改寫為join，這個動作就導(dǎo)致SQL在執(zhí)行計劃中會優(yōu)先選擇小表作為驅(qū)動表，而并不一定是左表t1作為驅(qū)動表。

root@mysql 11:26: [db1]> explain select * from t1 left join t2 on t1.uname=t2.uname and t2.uage>12; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+ | 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 5 | 100.00 | NULL | | 1 | SIMPLE | t2 | NULL | ALL | idx_uname | NULL | NULL | NULL | 3 | 100.00 | Using where; Using join buffer (Block Nested Loop) | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+ 2 rows in set, 1 warning (0.00 sec)root@mysql 11:26: [db1]> show warnings\G *************************** 1. row ***************************Level: NoteCode: 1003 Message: /* select#1 */ select `db1`.`t1`.`uid` AS `uid`,`db1`.`t1`.`uname` AS `uname`,`db1`.`t1`.`is_delete` AS `is_delete`,`db1`.`t2`.`id` AS `id`,`db1`.`t2`.`uname` AS `uname`,`db1`.`t2`.`uage` AS `uage` from `db1`.`t1` left join `db1`.`t2` on(((`db1`.`t2`.`uname` = `db1`.`t1`.`uname`) and (`db1`.`t2`.`uage` > 12))) where 1 1 row in set (0.00 sec)root@mysql 11:26: [db1]> root@mysql 11:26: [db1]> root@mysql 11:26: [db1]> explain select * from t1 left join t2 on t1.uname=t2.uname where t2.uage>12; +----+-------------+-------+------------+------+---------------+-----------+---------+--------------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+-----------+---------+--------------+------+----------+-------------+ | 1 | SIMPLE | t2 | NULL | ALL | idx_uname | NULL | NULL | NULL | 3 | 33.33 | Using where | | 1 | SIMPLE | t1 | NULL | ref | idx_uname | idx_uname | 43 | db1.t2.uname | 1 | 100.00 | NULL | +----+-------------+-------+------------+------+---------------+-----------+---------+--------------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)root@mysql 11:26: [db1]> show warnings\G *************************** 1. row ***************************Level: NoteCode: 1003 Message: /* select#1 */ select `db1`.`t1`.`uid` AS `uid`,`db1`.`t1`.`uname` AS `uname`,`db1`.`t1`.`is_delete` AS `is_delete`,`db1`.`t2`.`id` AS `id`,`db1`.`t2`.`uname` AS `uname`,`db1`.`t2`.`uage` AS `uage` from `db1`.`t1` join `db1`.`t2` where ((`db1`.`t1`.`uname` = `db1`.`t2`.`uname`) and (`db1`.`t2`.`uage` > 12)) 1 row in set (0.00 sec)

1.3 left join … where … is null 的使用

基于以上的案例，我們可以得出以下結(jié)論：

1）left join會返回左表所有記錄，右表滿足過濾條件記錄正常反饋，不滿足記錄返回NULL處理，join只會返回兩表均滿足過濾條件的記錄

2）left join把過濾條件寫在on條件上時才是我們通俗理解上的left join，而left join中將表過濾條件寫在where上時，MySQL會把left join改寫為join。

3）left join關(guān)聯(lián)查詢時，表的驅(qū)動順序是確定的，左表作為驅(qū)動表與右表進(jìn)行關(guān)聯(lián)查詢，但是若MySQL優(yōu)化器將left join改寫為join的情況下，MySQL就會優(yōu)先選擇小表作為驅(qū)動表進(jìn)行關(guān)聯(lián)查詢，一定程度上提升了SQL的執(zhí)行效率。

但是，對于第一/二條，將過濾條件放在where條件上的時候，MySQL優(yōu)化器就一定會將left join 改寫為join嗎？left join一定會返回左表全部記錄嗎？答案顯然是“不一定的”，以下就為大家展示一個特例。

root@mysql 11:42: [db1]> select * from t1 left join t2 on t1.uname=t2.uname; +-----+-------+-----------+------+-------+------+ | uid | uname | is_delete | id | uname | uage | +-----+-------+-----------+------+-------+------+ | 1 | aa | 0 | 1 | aa | 12 | | 1 | aa | 0 | 2 | aa | 13 | | 3 | cc | 0 | 5 | cc | 12 | | 2 | bb | 0 | NULL | NULL | NULL | | 4 | dd | 0 | NULL | NULL | NULL | | 5 | ee | 0 | NULL | NULL | NULL | +-----+-------+-----------+------+-------+------+ 6 rows in set (0.00 sec)root@mysql 11:41: [db1]> explain select * from t1 left join t2 on t1.uname=t2.uname where t2.uage is null; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+ | 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 5 | 100.00 | NULL | | 1 | SIMPLE | t2 | NULL | ALL | idx_uname | NULL | NULL | NULL | 3 | 33.33 | Using where; Using join buffer (Block Nested Loop) | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+ 2 rows in set, 1 warning (0.00 sec)root@mysql 11:41: [db1]> show warnings\G *************************** 1. row ***************************Level: NoteCode: 1003 Message: /* select#1 */ select `db1`.`t1`.`uid` AS `uid`,`db1`.`t1`.`uname` AS `uname`,`db1`.`t1`.`is_delete` AS `is_delete`,`db1`.`t2`.`id` AS `id`,`db1`.`t2`.`uname` AS `uname`,`db1`.`t2`.`uage` AS `uage` from `db1`.`t1` left join `db1`.`t2` on((`db1`.`t2`.`uname` = `db1`.`t1`.`uname`)) where isnull(`db1`.`t2`.`uage`) 1 row in set (0.00 sec)root@mysql 11:41: [db1]> select * from t1 left join t2 on t1.uname=t2.uname where t2.uage is null; +-----+-------+-----------+------+-------+------+ | uid | uname | is_delete | id | uname | uage | +-----+-------+-----------+------+-------+------+ | 2 | bb | 0 | NULL | NULL | NULL | | 4 | dd | 0 | NULL | NULL | NULL | | 5 | ee | 0 | NULL | NULL | NULL | +-----+-------+-----------+------+-------+------+ 3 rows in set (0.00 sec)

可以看到，我們使用left join，但是where 過濾條件是右表某些字段is null的查詢時。首先從執(zhí)行計劃來看，MySQL優(yōu)化器并沒有將left join改寫為join；然后從結(jié)果返回來看，可以看到該SQL僅僅返回了滿足過濾條件的記錄，并沒有返回左表全部記錄。

對于left join，過濾條件是右表某字段is null的情況是一個特例，而且這種寫法經(jīng)常被DBA同學(xué)用來做業(yè)務(wù)上一些not in/not exists的改寫優(yōu)化。

1.4 結(jié)論

1）left join會返回左表所有記錄，右表滿足過濾條件記錄正常反饋，不滿足記錄返回NULL處理，join只會返回兩表均滿足過濾條件的記錄

2）left join把過濾條件寫在on條件上時才是我們通俗理解上的left join，而left join中將表過濾條件寫在where上時，MySQL會把left join改寫為join。

3）left join且where過濾條件為右表某字段is null時屬于一個特例，該情況下MySQL不會將left join改寫為join，從而其表關(guān)聯(lián)查詢的順序也只能是左表作為驅(qū)動表與右表進(jìn)行關(guān)聯(lián)查詢。

4）left join關(guān)聯(lián)查詢時，表的驅(qū)動順序是確定的，左表作為驅(qū)動表與右表進(jìn)行關(guān)聯(lián)查詢，但是若MySQL優(yōu)化器將left join改寫為join的情況下，MySQL就會優(yōu)先選擇小表作為驅(qū)動表進(jìn)行關(guān)聯(lián)查詢，一定程度上提升了SQL的執(zhí)行效率。

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