在查询“SELECT 1 ...”中使用“LIMIT 1”是否有意义？
是的，你可以在SELECT 1中使用LIMIT 1。
假设你正在使用SELECT 1，而你的表有数十亿条记录。在这种情况下，它会打印1亿次。
SELECT 1的语法如下所示 −
SELECT 1 FROM yourTableName;Suppose, you are using LIMIT 1 and your table has billions of records. This case, it will print 1 only once.
The syntax of SELECT 1 with LIMIT 1 is as follows −
SELECT 1 FROM yourTableName LIMIT 1;To understand the above syntax, let us create a table. The query to create a table is as follows −
mysql> create table Select1AndLimit1Demo
-> (
-> Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
-> Name varchar(20)
-> );
Query OK, 0 rows affected (1.99 sec)使用插入命令在表中插入一些记录。查询如下 −
mysql> insert into Select1AndLimit1Demo(Name) values('John');
Query OK, 1 row affected (0.21 sec)
mysql> insert into Select1AndLimit1Demo(Name) values('Carol');
Query OK, 1 row affected (0.14 sec)
mysql> insert into Select1AndLimit1Demo(Name) values('Sam');
Query OK, 1 row affected (0.11 sec)
mysql> insert into Select1AndLimit1Demo(Name) values('Bob');
Query OK, 1 row affected (0.18 sec)
mysql> insert into Select1AndLimit1Demo(Name) values('David');
Query OK, 1 row affected (0.14 sec)
mysql> insert into Select1AndLimit1Demo(Name) values('Mike');
Query OK, 1 row affected (0.20 sec)
mysql> insert into Select1AndLimit1Demo(Name) values('Maxwell');
Query OK, 1 row affected (0.11 sec)Display all records from the table using a select statement. The query is as follows −
mysql> select *from Select1AndLimit1Demo;输出
+----+---------+
| Id | Name |
+----+---------+
| 1 | John |
| 2 | Carol |
| 3 | Sam |
| 4 | Bob |
| 5 | David |
| 6 | Mike |
| 7 | Maxwell |
+----+---------+
7 rows in set (0.00 sec)这是SELECT 1的案例。查询如下 −
mysql> select 1 from Select1AndLimit1Demo;输出
+---+
| 1 |
+---+
| 1 |
| 1 |
| 1 |
| 1 |
| 1 |
| 1 |
| 1 |
+---+
7 rows in set (0.00 sec)Above, we have a table with 7 records. Therefore, the output is 7 times 1.
Let us now see the case of SELECT 1 with LIMIT 1. The query is as follows −
mysql> select 1 from Select1AndLimit1Demo limit 1;以下是输出的结果，只显示值为1一次 −
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.00 sec)上面，我们的表有7条记录。我们得到了1乘以1，因为我们使用了LIMIT 1。
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