看數據庫連接池源碼，發現連接池的參數validationQuery(SQL查詢,用來驗證從連接池取出的連接)設置的值為"SELECT 1"，之前很少用這種寫法，于是

google一下，總結如下:?

以seven_user表為例，如下是建表語句：

CREATE TABLE `smily_test` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`version` int(4) NOT NULL DEFAULT '0',
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;

select 1；
select 1 from smily_test；
select id from smily_test；
select version from smily_test；
select * from smily_test；

?以上sql語句執行的結果如下：

mysql> select 1;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.00 sec)

mysql> select 1 from smily_test;
+---+
| 1 |
+---+
| 1 |
| 1 |
| 1 |
| 1 |
+---+
4 rows in set (0.00 sec)

mysql> select id from smily_test;
+----+
| id |
+----+
| 1 |
| 2 |
| 3 |
| 4 |
+----+
4 rows in set (0.00 sec)

mysql> select version from smily_test;
+---------+
| version |
+---------+
| 1 |
| 2 |
| 3 |
| 4 |
+---------+
4 rows in set (0.00 sec)

mysql> select * from smily_test;

+----+---------+
| id | version |
+----+---------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
+----+---------+
4 rows in set (0.00 sec)

用show profiles查看這些sql語句的執行效率；

mysql> show profiles; +----------+------------+--------------------------------+ | Query_ID | Duration | Query | +----------+------------+--------------------------------+ | 1 | 0.00017700 | select 1 | | 2 | 0.00013400 | select 1 from smily_test | | 3 | 0.00011000 | select id from smily_test | | 4 | 0.00010500 | select version from smily_test | | 5 | 0.00011700 | select * from smily_test | +----------+------------+--------------------------------+ 5 rows in set (0.00 sec)

作用上來說是沒有差別的，都是查看是否有記錄，一般是作條件用的.

看得出執行效率為： ?select 1 > select 1 from table > select anycolum from table > select * from table?

再來看看 count(*) 和 count(1)的效率。

+----------+------------+------------------------------------+ | Query_ID | Duration | Query | +----------+------------+------------------------------------+ | 7 | 0.00037200 | select count(1) from smily_test | | 8 | 0.00034500 | select count(*) from smily_test | +----------+------------+------------------------------------+

?

轉載于:https://www.cnblogs.com/happySmily/p/5939704.html

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