MySQL中count(*)、count(1)和count(col)的区别汇总

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)

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)

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) |
+-----------+
|   3 |
+-----------+
1 row in set (0.00 sec)
   "table": {
    "table_name": "test1",
    "access_type": "index",
    "key": "idx_c1",
    "used_key_parts": [
     "c1"
    ],
    "key_length": "33",