Mysql中的varchar型別轉換成int型別

1.實戰案例

1.1 student表結構

mysql> desc student;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | int(11)     | YES  |     | NULL    |       |
 

| name     | varchar(20) | YES  |     | NULL    |       |
| address  | varchar(20) | YES  |     | NULL    |       |
| score    | varchar(3)  | YES  |     | NULL    |       |
| birthday | datetime    | YES  |     | NULL    |       |
| age      | int(10)     | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
 

6 rows in set (0.00 sec)

注意都score我使用的是varchar型別，這裡需要將其轉換成int型。

1.2 插入資料

insert into student values(1,'lawson','',99,'1996-09-17',22);
insert into student values(2,'ting','anhui_huaibei','','1996-09-17',22);

mysql> select * from student;
+------+--------+---------------+-------+---------------------+------+
 

| id   | name   | address       | score | birthday            | age  |
+------+--------+---------------+-------+---------------------+------+
|    1 | lawson |               | 99    | 1996-09-17 00:00:00 |   22 |
|    2 | ting   | anhui_huaibei |       | 1996-09-17 00:00:00 |   22 |
+------+--------+---------------+-------+---------------------+------+
2 rows in set (0.00 sec)

1. 3 使用case when end句型 + cast強轉

mysql> select
    -> id
    -> ,name
    -> ,case when score='' then 0
    -> else cast(score as signed) end score
    -> from student;
+------+--------+-------+
| id   | name   | score |
+------+--------+-------+
|    1 | lawson |    99 |
|    2 | ting   |     0 |
+------+--------+-------+
2 rows in set (0.03 sec)

對比沒有使用cast語句

mysql> select id,name,score from student;
+------+--------+-------+
| id   | name   | score |
+------+--------+-------+
|    1 | lawson | 99    |
|    2 | ting   |       |
+------+--------+-------+
2 rows in set (0.00 sec)

2.注意事項

• 使用cast句型時，必須符合cast(score as signed)句型，這裡的signed等價於int。但是不能寫成cast(score as int)，否則會報錯。但是singed並不是mysql中的基本型別，比如使用其建立表時會出錯。

mysql> create table t (id int);
Query OK, 0 rows affected (0.22 sec)

mysql> create table t1 (id signed);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'signed)' at line 1