《MYSQL數(shù)據(jù)庫(kù)MySQL中無GROUP BY情況下直接使用HAVING語句的問題探究》要點(diǎn)：
本文介紹了MYSQL數(shù)據(jù)庫(kù)MySQL中無GROUP BY情況下直接使用HAVING語句的問題探究，希望對(duì)您有用。如果有疑問，可以聯(lián)系我們。
今天有同學(xué)給我反應(yīng),有一張表,id是主鍵,這樣的寫法可以返回一條記錄：MYSQL必讀
“SELECT * FROM t HAVING id=MIN(id);”
?? 但是只是把MIN換成MAX,這樣返回就是空了：MYSQL必讀
“SELECT * FROM t HAVING id=MAX(id);”
?? 這是為什么呢?MYSQL必讀
?? 我們先來做個(gè)試驗(yàn),驗(yàn)證這種情況.MYSQL必讀
?? 這是表結(jié)構(gòu),初始化兩條記錄,然后試驗(yàn)：MYSQL必讀
root@localhost : plx 10:25:10> show create table t2G
*************************** 1. row ***************************
Table: t2
Create Table: CREATE TABLE `t2` (
`a` int(11) DEFAULT NULL,
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8
root@localhost : plx 10:25:15> select * from t2;
+------+----+
| a | id |
+------+----+
| 1 | 1 |
| 1 | 3 |
+------+----+
2 rows in set (0.00 sec)
root@localhost : plx 10:25:20> SELECT * FROM t2 HAVING id=MIN(id);
+------+----+
| a | id |
+------+----+
| 1 | 1 |
+------+----+
1 row in set (0.00 sec)
root@localhost : plx 10:25:30> SELECT * FROM t2 HAVING id=MAX(id);
Empty set (0.00 sec)
?? 初看之下,好像真的是這樣哎,怎么會(huì)這樣呢?MYSQL必讀
?? 我再試一下,把a(bǔ)字段改一個(gè)為10,然后試下a字段：MYSQL必讀
root@localhost : plx 10:26:58> select * from t2; +------+----+ | a | id | +------+----+ | 10 | 1 | | 1 | 3 | +------+----+ 2 rows in set (0.00 sec) root@localhost : plx 10:28:20> SELECT * FROM t2 HAVING a=MAX(a); +------+----+ | a | id | +------+----+ | 10 | 1 | +------+----+ 1 row in set (0.00 sec) root@localhost : plx 10:28:28> SELECT * FROM t2 HAVING a=MIN(a); Empty set (0.00 sec)
?? 我擦,這回MAX能返回,MIN不能了,這又是為啥呢?MYSQL必讀
?? 旁白MYSQL必讀
?? 一般來說,HAVING子句是配合GROUP BY使用的,單獨(dú)使用HAVING本身是不符合規(guī)范的,MYSQL必讀
?? 但是MySQL會(huì)做一個(gè)重寫,加上一個(gè)GROUP BY NULL,”SELECT * FROM t HAVING id=MIN(id)”會(huì)被重寫為”SELECT * FROM t GROUP BY NULL HAVING id=MIN(id)”,這樣語法就符合規(guī)范了.MYSQL必讀
?? 繼續(xù)……MYSQL必讀
?? 但是,這個(gè) GROUP BY NULL 會(huì)產(chǎn)生什么結(jié)果呢?經(jīng)過查看代碼和試驗(yàn),可以證明,GROUP BY NULL 等價(jià)于 LIMIT 1：MYSQL必讀
root@localhost : plx 10:25:48> SELECT * FROM t2 GROUP BY NULL; +------+----+ | a | id | +------+----+ | 10 | 1 | +------+----+ 1 row in set (0.00 sec)
?? 也就是說,GROUP BY NULL 以后,只會(huì)有一個(gè)分組,里面就是第一行數(shù)據(jù).MYSQL必讀
?? 但是如果這樣,MIN、MAX結(jié)果應(yīng)該是一致的,那也不應(yīng)該MAX和MIN一個(gè)有結(jié)果,一個(gè)沒結(jié)果啊,這是為什么呢,再做一個(gè)測(cè)試.MYSQL必讀
?? 修改一下數(shù)據(jù),然后直接查看MIN/MAX的值：MYSQL必讀
root@localhost : plx 10:26:58> select * from t2; +------+----+ | a | id | +------+----+ | 10 | 1 | | 1 | 3 | +------+----+ 2 rows in set (0.00 sec) root@localhost : plx 10:27:04> SELECT * FROM t2 GROUP BY NULL; +------+----+ | a | id | +------+----+ | 10 | 1 | +------+----+ 1 row in set (0.00 sec) root@localhost : plx 10:30:21> SELECT MAX(a),MIN(a),MAX(id),MIN(id) FROM t2 GROUP BY NULL; +--------+--------+---------+---------+ | MAX(a) | MIN(a) | MAX(id) | MIN(id) | +--------+--------+---------+---------+ | 10 | 1 | 3 | 1 | +--------+--------+---------+---------+ 1 row in set (0.00 sec)
?? 是不是發(fā)現(xiàn)問題了?MYSQL必讀
?? MAX/MIN函數(shù)取值是全局的,而不是LIMIT 1這個(gè)分組內(nèi)的.MYSQL必讀
?? 因此,當(dāng)GROUP BY NULL的時(shí)候,MAX/MIN函數(shù)是取所有數(shù)據(jù)里的最大和最小值!MYSQL必讀
?? 所以啊,”SELECT * FROM t HAVING id=MIN(id)”本質(zhì)上是”SELECT * FROM t HAVING id=1″, 就能返回一條記錄,而”SELECT * FROM t HAVING id=MAX(id)”本質(zhì)上是”SELECT * FROM t HAVING id=3″,當(dāng)然沒有返回記錄,這就是問題的根源.MYSQL必讀
?? 測(cè)試一下GROUP BY a,這樣就對(duì)了,每個(gè)分組內(nèi)只有一行,所以MAX/MIN一樣大,這回是取得組內(nèi)最大和最小值.MYSQL必讀
root@localhost : plx 11:29:49> SELECT MAX(a),MIN(a),MAX(id),MIN(id) FROM t2 GROUP BY a; +--------+--------+---------+---------+ | MAX(a) | MIN(a) | MAX(id) | MIN(id) | +--------+--------+---------+---------+ | 1 | 1 | 3 | 3 | | 10 | 10 | 5 | 5 | +--------+--------+---------+---------+ 2 rows in set (0.00 sec)
?? GROUP BY NULL時(shí)MAX/MIN的行為,是這個(gè)問題的本質(zhì),所以啊,盡量使用標(biāo)準(zhǔn)語法,玩花樣SQL之前,一定要搞清楚它的行為是否與理解的一致.MYSQL必讀
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