万答#11，MySQL中char与varchar有什么区别

2021-08-04 00:14:05

1.实验场景

GreatSQL 8.0.25 InnoDB

2.实验测试

2.1 区别

参数	char	varchar
长度是否可变	定长	变长
存储容量	0 ~ 255	0 ~ 65,535

2.2 建测试表

CREATE TABLE vc (v VARCHAR(4), c CHAR(4));

2.3 未超出设定值测试

字段V、C都写入一个4+空格的字符

[root@GreatSQL][test]> INSERT INTO vc VALUES ('4 ', '4 ');

[root@GreatSQL][test]> SELECT CONCAT('(', v, ')'), CONCAT('(', c, ')') FROM vc;
+---------------------+---------------------+
| CONCAT('(', v, ')') | CONCAT('(', c, ')') |
+---------------------+---------------------+
| (4 )                | (4)                 |
+---------------------+---------------------+
1 rows in set (0.00 sec)

测试结果，char的长度维持不变，占了2个字符，varchar空格长度变了，占了一个字符。

2.4 超出设定值测试

当写入长度大于设定长度时候，出现报错

[root@GreatSQL][test]>INSERT INTO vc VALUES ('123456', '123456');
ERROR 1406 (22001): Data too long for column 'v' at row 1

调整sql_mode，再写入的时候，自动截取限制容量内的内容

[root@GreatSQL][test]>set session sql_mode="ONLY_FULL_GROUP_BY,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION"

[root@GreatSQL][test]>INSERT INTO vc VALUES ('123456', '123456');            
Query OK, 1 row affected, 2 warnings (0.02 sec)

[root@GreatSQL][test]>select * from vc;
+------+------+
| v    | c    |
+------+------+
| 1234 | 1234 |
+------+------+
1 row in set (0.00 sec)

2.5 字段长度测试

创建表，设置CHAR长度为256, 结果提示错误

[root@GreatSQL][test]>CREATE TABLE vc (v VARCHAR(255), c CHAR(256));
ERROR 1074 (42000): Column length too big for column 'c' (max = 255); use BLOB or TEXT instead