select 1 from ...详解

select 1 from table，不查询具体的列的内容，只要有值就显示为1.
实例：
mysql>create table t1(id int);
mysql>insert into  t1 values(2);
mysql>insert into  t1 values(3);
mysql>select * from t1;
+------+
| id   |
+------+
|    2 |
|    3 |
+------+
2 rows in set (0.00 sec)
mysql>select 1 from t1;
+---+
| 1 |
+---+
| 1 |
| 1 |
+---+
2 rows in set (0.00 sec)
可以看到，不是具体的值，有值便显示为1，列名也显示为1.