查看錶結構：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | mysql> show create table coupon_use_test \G*************************** 1. row *************************** Table: coupon_use_testCreate Table: CREATE TABLE `coupon_use_test` ( `id` int(11) NOT NULL DEFAULT '0', `user_id` varchar(40) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL, `coupon_code` varchar(40) CHARACTER SET utf8 COLLATE utf8_bin NOT NULL DEFAULT '', `status` varchar(2) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT '00', `use_time` datetime DEFAULT NULL, `remark1` varchar(200) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL, `remark2` varchar(200) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL, `remark3` varchar(200) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL, `create_time` timestamp NULL DEFAULT CURRENT_TIMESTAMP, `create_user_id` varchar(128) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL, `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, KEY `idx_create_time` (`create_time`)) ENGINE=InnoDB DEFAULT CHARSET=utf81 row in set (0.00 sec) |
查看create_time字段爲空的行數
1 2 | mysql> select * from coupon_use_test where create_time is null;Empty set (0.00 sec) |
把id爲1的記錄create_time改爲空
1 2 3 4 5 6 7 8 9 10 | mysql> update coupon_use_test set create_time = null where id = 1;Query OK, 1 row affected (6.56 sec)Rows matched: 1 Changed: 1 Warnings: 0mysql> select count(*) from coupon_use_test where create_time is null;+----------+| count(*) |+----------+| 1 |+----------+1 row in set (0.00 sec) |
count(*)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | mysql> select count(*) from coupon_use_test;+----------+| count(*) |+----------+| 1800000 |+----------+1 row in set (0.69 sec)mysql> explain select count(*) from coupon_use_test;+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+| 1 | SIMPLE | coupon_use_test | NULL | index | NULL | idx_create_time | 5 | NULL | 1771323 | 100.00 | Using index |+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+1 row in set, 1 warning (0.00 sec) |
可以看到count(*)走了create_time字段的索引idx_create_time
count(1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | mysql> select count(1) from coupon_use_test;+----------+| count(1) |+----------+| 1800000 |+----------+1 row in set (0.63 sec)mysql> explain select count(1) from coupon_use_test;+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+| 1 | SIMPLE | coupon_use_test | NULL | index | NULL | idx_create_time | 5 | NULL | 1771323 | 100.00 | Using index |+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+1 row in set, 1 warning (0.00 sec) |
count(create_time)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | mysql> select count(create_time) from coupon_use_test;+--------------------+| count(create_time) |+--------------------+| 1799999 |+--------------------+1 row in set (0.73 sec)mysql> explain select count(create_time) from coupon_use_test;+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+| 1 | SIMPLE | coupon_use_test | NULL | index | NULL | idx_create_time | 5 | NULL | 1771323 | 100.00 | Using index |+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+1 row in set, 1 warning (0.00 sec) |
count(*)和 count(1)可以查詢全表總行數， count(create_time)查詢到的行數不包括null。
count(1) 與 count(*) 比較 :
1> 如果數據表沒有主鍵，那麼 count(1) 比 count(*) 快
2> 如果有主鍵的話，那主鍵 (聯合主鍵) 作爲 count條件也比 count(*) 要快
3> 如果你的表只有一個字段的話那 count(*) 就是最快
在不加 WHERE 限制條件的情況下，COUNT(*) 與 COUNT(COL) 基本可以認爲是等價的，但是在有 WHERE 限制條件的情況下，COUNT(*) 會比 COUNT(COL) 快非常多
COUNT(*) 通常是對主鍵進行索引掃描，而COUNT(COL)就不一定了，另外前者是統計表中的所有符合的紀錄總數，而後者是計算表中所有符合的COL的紀錄數
count(*) 與 count(1) 兩者比較，主要還是要取決於 count(1) 所相對應的數據字段，如果count(1)是聚索引 id 那肯定是count(1)快，但是差的很小，因爲 count(*) 自動會優化指定到那一個字段，所以沒必要去count(?)用count(*)sql會自動完成優化
1> 任何情況下 SELECT COUNT(*) FROM tablename 是最優選擇
2> 儘量減少 SELECT COUNT(*) FROM tablename WHERE COL = 'value’ 這種查詢
3> 杜絕 SELECT COUNT(COL) FROM tablename 的出現
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