MySQL中count(*)、count(1)和count(col)的区别汇总
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2020-09-09
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MySQL中中count(*)、、count(1)和和count(col)的区别汇总的区别汇总
count()函数是用来统计表中记录的一个函数，返回匹配条件的行数，下面这篇文章主要给大家总结介绍了关于
MySQL中count(*)、count(1)和count(col)的区别，文中通过示例代码介绍的非常详细，需要的朋友可以参考下。
前言前言
count函数是用来统计表中或数组中记录的一个函数，count(*) 它返回检索行的数目， 不论其是否包含 NULL值。最近感觉大
家都在讨论count的区别，那么我也写下吧：欢迎留言讨论，话不多说了，来一起看看详细的介绍吧。
1、表结构：、表结构：
dba_jingjing@3306>[rds_test]>CREATE TABLE `test_count` (
-> `c1` varchar(10) DEFAULT NULL,
-> `c2` varchar(10) DEFAULT NULL,
-> KEY `idx_c1` (`c1`)
-> ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
Query OK, 0 rows affected (0.11 sec)
2、插入测试数据：、插入测试数据：
dba_jingjing@3306>[rds_test]>insert into test_count values(1,10);
Query OK, 1 row affected (0.03 sec)
dba_jingjing@3306>[rds_test]>insert into test_count values(abc,null);
ERROR 1054 (42S22): Unknown column 'abc' in 'field list'
dba_jingjing@3306>[rds_test]>insert into test_count values('abc',null);
Query OK, 1 row affected (0.04 sec)
dba_jingjing@3306>[rds_test]>insert into test_count values(null,null);
Query OK, 1 row affected (0.04 sec)
dba_jingjing@3306>[rds_test]>insert into test_count values('368rhf8fj',null);
Query OK, 1 row affected (0.03 sec)
dba_jingjing@3306>[rds_test]>select * from test_count;
+-----------+------+
| c1 | c2 |
+-----------+------+
| 1 | 10 |
| abc | NULL |
| NULL | NULL |
| 368rhf8fj | NULL |
+-----------+------+
4 rows in set (0.00 sec)
测试：测试：
dba_jingjing@3306>[rds_test]>select count(*) from test_count;
+----------+
| count(*) |
+----------+
| 4 |
+----------+
1 row in set (0.00 sec)
EXPLAIN: {
"query_block": {
"select_id": 1,
"message": "Select tables optimized away"
1 row in set, 1 warning (0.00 sec)
dba_jingjing@3306>[rds_test]>select count(1) from test_count;
+----------+
| count(1) |
+----------+
| 4 |
+----------+
1 row in set (0.00 sec)
EXPLAIN: {
"query_block": {
"select_id": 1,
"message": "Select tables optimized away"
1 row in set, 1 warning (0.00 sec)
dba_jingjing@3306>[rds_test]>select count(c1) from test_count;
+-----------+
| count(c1) |
+-----------+
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