技术分享 | check(col_name＜＞‘‘)为何把空格拒之门外

1、问题描述

前两天在群里看到同事反馈一个空格问题，大致现象如下：

mysql> select @@version;
+-----------+
| @@version |
+-----------+
| 8.0.25    |
+-----------+
1 row in set (0.00 sec)

mysql> create table t1(
    -> c1 int,
    -> c2 varchar(4) check(c2<>'')  #单引号之间无空格
    -> )engine=innodb;
Query OK, 0 rows affected (0.21 sec)

mysql> insert into t1 select 1,'  ';  #c2字段插入两个空格
ERROR 3819 (HY000): Check constraint 't1_chk_1' is violated.

check定义c2<>''，往c2字段插入空格，提示违反check约束。

为什么insert语句中的' '(单引号之间有一个或多个空格)会被判断为''(单引号之间无空格)，导致插入失败？

2、涉及知识

2.1、Stored and Retrieved

https://dev.mysql.com/doc/refman/8.0/en/char.html

When CHAR values are stored, they are right-padded with spaces to the specified length. When CHAR values are retrieved, trailing spaces are removed unless the PAD_CHAR_TO_FULL_LENGTH SQL mode is enabled.

VARCHAR values are not padded when they are stored. Trailing spaces are retained when values are stored and retrieved, in conformance with standard SQL.

CHAR(N)：当插入的字符数小于N，它会在字符串的右边补充空格，直到总字符数达到N再进行存储；当查询返回数据时默认会将字符串尾部的空格去掉，除非SQL_MODE设置PAD_CHAR_TO_FULL_LENGTH(手册显示8.0.13 deprecated，8.0.25还能使用)。

VARCHAR(N)：当插入的字符数小于N，它不会在字符串的右边补充空格，insert内容原封不动的进行存储；如果原本字符串右边有空格，在存储和查询返回时都会保留空格。

2.2、Collation Pad Attribute

https://dev.mysql.com/doc/refman/8.0/en/char.html

Values in CHAR, VARCHAR, and TEXT columns are sorted and compared according to the character set collation assigned to the column.

MySQL collations have a pad attribute of PAD SPACE, other than Unicode collations based on UCA 9.0.0 and higher, which have a pad attribute of NO PAD.

对于CHAR、VARCHAR、TEXT字段，排序和比较运算依赖字段上的Collation，Collation的Pad属性控制字符串尾部空格处理方式。 可以通过INFORMATION_SCHEMA.COLLATIONS表，查看Collation所使用的Pad属性：

mysql> select collation_name,pad_attribute from information_schema.collations;
+----------------------------+---------------+
| collation_name             | pad_attribute |
+----------------------------+---------------+
| armscii8_general_ci        | PAD SPACE     |
...
| utf8mb4_0900_bin           | NO PAD        |
+----------------------------+---------------+
272 rows in set (0.01 sec)

2.3、Trailing Space Handling in Comparisons

https://dev.mysql.com/doc/refman/8.0/en/charset-binary-collations.html#charset-binary-collations-trailing-space-comparisons

For nonbinary strings (CHAR, VARCHAR, and TEXT values), the string collation pad attribute determines treatment in comparisons of trailing spaces at the end of strings:

• For PAD SPACE collations, trailing spaces are insignificant in comparisons; strings are compared without regard to trailing spaces.

• NO PAD collations treat trailing spaces as significant in comparisons, like any other character.

"Comparison" in this context does not include the LIKE pattern-matching operator, for which trailing spaces are significant, regardless of collation.

PAD SPACE：在排序和比较运算中，忽略字符串尾部空格。 NO PAD：在排序和比较运算中，字符串尾部空格当成普通字符，不能忽略。

3、问题解决

以下操作基于MySQL 8.0.25 社区版

3.1、查看字段使用的Collation

mysql> show full fields in t1;
+-------+------------+--------------------+------+-----+---------+-------+---------------------------------+---------+
| Field | Type       | Collation          | Null | Key | Default | Extra | Privileges                      | Comment |
+-------+------------+--------------------+------+-----+---------+-------+---------------------------------+---------+
| c1    | int        | NULL               | YES  |     | NULL    |       | select,insert,update,references |         |
| c2    | varchar(4) | utf8mb4_unicode_ci | YES  |     | NULL    |       | select,insert,update,references |         |
+-------+------------+--------------------+------+-----+---------+-------+---------------------------------+---------+
2 rows in set (0.00 sec)

c2列的Collation是utf8mb4_unicode_ci。

3.2、查看Collation的Pad属性

mysql> select COLLATION_NAME,PAD_ATTRIBUTE from INFORMATION_SCHEMA.COLLATIONS where COLLATION_NAME in('utf8mb4_unicode_ci','utf8mb4_0900_ai_ci');
+--------------------+---------------+
| COLLATION_NAME     | PAD_ATTRIBUTE |
+--------------------+---------------+
| utf8mb4_0900_ai_ci | NO PAD        |
| utf8mb4_unicode_ci | PAD SPACE     |
+--------------------+---------------+
1 row in set (0.00 sec)

utf8mb4_unicode_ci的Pad属性是PAD SPACE，由2.3可知c2列在排序和比较运算中，忽略字符串尾部空格。

因此check比较时，会将插入的' '中的空格忽略，显然忽略空格后和check约束存在冲突，插入失败。

mysql> SET NAMES utf8mb4 COLLATE utf8mb4_unicode_ci;
Query OK, 0 rows affected (0.00 sec)

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

3.3、如何让check约束按常规逻辑生效

这里的常规是指空格就是空格，不应该把空格忽略。只需将c2字段修改为NO PAD的Collation后，就能将空格正常插入：

mysql> insert into t1 select 1,'  ';  #c2字段插入两个空格
ERROR 3819 (HY000): Check constraint 't1_chk_1' is violated.

mysql> alter table t1 modify c2 varchar(4) collate utf8mb4_0900_ai_ci;  #修改为NO PAD的Collation
Query OK, 0 rows affected (0.15 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> insert into t1 select 1,'  ';  #c2字段插入两个空格
Query OK, 1 row affected (0.12 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> insert into t1 select 1,'';  #''之间无空格
ERROR 3819 (HY000): Check constraint 't1_chk_1' is violated.

mysql> select c1,c2,hex(c2) from t1;
+------+------+---------+
| c1   | c2   | hex(c2) |
+------+------+---------+
|    1 |      | 2020    |
+------+------+---------+
1 row in set (0.01 sec)

4、扩展

4.1、如果c2列是CHAR类型，和前面的问题表现一样吗

一样。CHAR、VARCHAR、TEXT在做排序和比较运算时，都是依据列的Collation的Pad属性处理字符串尾部的空格。此时拿来做比较运算的字符串是insert中的内容。

4.2、WHERE条件中表现形式是怎样的

创建一张新表并插入数据

mysql> create table t3(
    -> c1 int,
    -> c2 char(4) collate utf8mb4_unicode_ci,
    -> c3 char(4) collate utf8mb4_0900_ai_ci,
    -> c4 varchar(4) collate utf8mb4_unicode_ci,
    -> c5 varchar(4) collate utf8mb4_0900_ai_ci
    -> )engine=innodb;
Query OK, 0 rows affected (0.29 sec)

mysql> insert into t3 select 1,'a','a','a','a';
Query OK, 1 row affected (0.09 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> insert into t3 select 2,'a ','a ','a ','a ';  #各列包含1个空格
Query OK, 1 row affected (0.20 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> insert into t3 select 3,'a   ','a   ','a  ','a  ';  #前两列3个空格，后两列2个空格
Query OK, 1 row affected (0.17 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> insert into t3 select 4,'a  ','a  ','a   ','a   ';  #前两列2个空格，后两列3个空格
Query OK, 1 row affected (0.14 sec)
Records: 1  Duplicates: 0  Warnings: 0

观察WHERE条件返回结果，CHAR类型的返回受PAD_CHAR_TO_FULL_LENGTH影响(参考2.1)

mysql> set sql_mode='';
Query OK, 0 rows affected (0.00 sec)

mysql> select *,hex(c2),hex(c3),hex(c4),hex(c5) from t3 where c2='a';
+------+------+------+------+------+---------+---------+----------+----------+
| c1   | c2   | c3   | c4   | c5   | hex(c2) | hex(c3) | hex(c4)  | hex(c5)  |
+------+------+------+------+------+---------+---------+----------+----------+
|    1 | a    | a    | a    | a    | 61      | 61      | 61       | 61       |
|    2 | a    | a    | a    | a    | 61      | 61      | 6120     | 6120     |
|    3 | a    | a    | a    | a    | 61      | 61      | 612020   | 612020   |
|    4 | a    | a    | a    | a    | 61      | 61      | 61202020 | 61202020 |
+------+------+------+------+------+---------+---------+----------+----------+
4 rows in set (0.00 sec)

c2 char->返回数据去掉字符串尾部的空格

c2 utf8mb4_unicode_ci->PAD SPACE->排序和比较运算，忽略字符串尾部空格

mysql> select *,hex(c2),hex(c3),hex(c4),hex(c5) from t3 where c3='a';
+------+------+------+------+------+---------+---------+----------+----------+
| c1   | c2   | c3   | c4   | c5   | hex(c2) | hex(c3) | hex(c4)  | hex(c5)  |
+------+------+------+------+------+---------+---------+----------+----------+
|    1 | a    | a    | a    | a    | 61      | 61      | 61       | 61       |
|    2 | a    | a    | a    | a    | 61      | 61      | 6120     | 6120     |
|    3 | a    | a    | a    | a    | 61      | 61      | 612020   | 612020   |
|    4 | a    | a    | a    | a    | 61      | 61      | 61202020 | 61202020 |
+------+------+------+------+------+---------+---------+----------+----------+
4 rows in set (0.01 sec)

c3 char->返回数据去掉字符串尾部的空格

c3 utf8mb4_0900_ai_ci->NO PAD->排序和比较运算，字符串尾部空格当成普通字符

mysql> select *,hex(c2),hex(c3),hex(c4),hex(c5) from t3 where c4='a';
+------+------+------+------+------+---------+---------+----------+----------+
| c1   | c2   | c3   | c4   | c5   | hex(c2) | hex(c3) | hex(c4)  | hex(c5)  |
+------+------+------+------+------+---------+---------+----------+----------+
|    1 | a    | a    | a    | a    | 61      | 61      | 61       | 61       |
|    2 | a    | a    | a    | a    | 61      | 61      | 6120     | 6120     |
|    3 | a    | a    | a    | a    | 61      | 61      | 612020   | 612020   |
|    4 | a    | a    | a    | a    | 61      | 61      | 61202020 | 61202020 |
+------+------+------+------+------+---------+---------+----------+----------+
4 rows in set (0.00 sec)

c4 varchar->返回数据保留插入时的空格

c4 utf8mb4_unicode_ci->PAD SPACE->排序和比较运算，忽略字符串尾部空格

mysql> select *,hex(c2),hex(c3),hex(c4),hex(c5) from t3 where c5='a';
+------+------+------+------+------+---------+---------+---------+---------+
| c1   | c2   | c3   | c4   | c5   | hex(c2) | hex(c3) | hex(c4) | hex(c5) |
+------+------+------+------+------+---------+---------+---------+---------+
|    1 | a    | a    | a    | a    | 61      | 61      | 61      | 61      |
+------+------+------+------+------+---------+---------+---------+---------+
1 row in set (0.00 sec)

c5 varchar->返回数据保留插入时的空格

c5 utf8mb4_0900_ai_ci->NO PAD->排序和比较运算，字符串尾部空格当成普通字符


mysql> set sql_mode='PAD_CHAR_TO_FULL_LENGTH';
Query OK, 0 rows affected, 1 warning (0.01 sec)

mysql> select *,hex(c2),hex(c3),hex(c4),hex(c5) from t3 where c2='a';
+------+------+------+------+------+----------+----------+----------+----------+
| c1   | c2   | c3   | c4   | c5   | hex(c2)  | hex(c3)  | hex(c4)  | hex(c5)  |
+------+------+------+------+------+----------+----------+----------+----------+
|    1 | a    | a    | a    | a    | 61202020 | 61202020 | 61       | 61       |
|    2 | a    | a    | a    | a    | 61202020 | 61202020 | 6120     | 6120     |
|    3 | a    | a    | a    | a    | 61202020 | 61202020 | 612020   | 612020   |
|    4 | a    | a    | a    | a    | 61202020 | 61202020 | 61202020 | 61202020 |
+------+------+------+------+------+----------+----------+----------+----------+
4 rows in set (0.00 sec)

c2 char->PAD_CHAR_TO_FULL_LENGTH->返回数据字符串右边补充空格

c2 utf8mb4_unicode_ci->PAD SPACE->排序和比较运算，忽略字符串尾部空格

mysql> select *,hex(c2),hex(c3),hex(c4),hex(c5) from t3 where c3='a';
Empty set (0.00 sec)

c3 char->PAD_CHAR_TO_FULL_LENGTH->返回数据字符串右边补充空格

c3 utf8mb4_0900_ai_ci->NO PAD->排序和比较运算，字符串尾部空格当成普通字符

1~4行c3列返回值都包含空格，且c3列的Collation是NO PAD，字符串尾部空格不能忽略，where过滤找不到记录

mysql> select *,hex(c2),hex(c3),hex(c4),hex(c5) from t3 where c4='a';
+------+------+------+------+------+----------+----------+----------+----------+
| c1   | c2   | c3   | c4   | c5   | hex(c2)  | hex(c3)  | hex(c4)  | hex(c5)  |
+------+------+------+------+------+----------+----------+----------+----------+
|    1 | a    | a    | a    | a    | 61202020 | 61202020 | 61       | 61       |
|    2 | a    | a    | a    | a    | 61202020 | 61202020 | 6120     | 6120     |
|    3 | a    | a    | a    | a    | 61202020 | 61202020 | 612020   | 612020   |
|    4 | a    | a    | a    | a    | 61202020 | 61202020 | 61202020 | 61202020 |
+------+------+------+------+------+----------+----------+----------+----------+
4 rows in set (0.00 sec)
c4 varchar->返回数据保留插入时的空格
c4 utf8mb4_unicode_ci->PAD SPACE->排序和比较运算，忽略字符串尾部空格

mysql> select *,hex(c2),hex(c3),hex(c4),hex(c5) from t3 where c5='a';
+------+------+------+------+------+----------+----------+---------+---------+
| c1   | c2   | c3   | c4   | c5   | hex(c2)  | hex(c3)  | hex(c4) | hex(c5) |
+------+------+------+------+------+----------+----------+---------+---------+
|    1 | a    | a    | a    | a    | 61202020 | 61202020 | 61      | 61      |
+------+------+------+------+------+----------+----------+---------+---------+
1 row in set (0.00 sec)

c5 varchar->返回数据保留插入时的空格

c5 utf8mb4_0900_ai_ci->NO PAD->排序和比较运算，字符串尾部空格当成普通字符

此时拿来做比较运算的字符串是Retrieved的内容，CHAR和VARCHAR返回数据时对字符串尾部的空格处理方式不同，并且PAD_CHAR_TO_FULL_LENGTH只影响CHAR类型。

4.3、对唯一索引的影响

https://dev.mysql.com/doc/refman/8.0/en/char.html

For those cases where trailing pad characters are stripped or comparisons ignore them, if a column has an index that requires unique values, inserting into the column values that differ only in number of trailing pad characters results in a duplicate-key error. For example, if a table contains 'a', an attempt to store 'a ' causes a duplicate-key error.

如果存在唯一索引(单列、字符类型)，插入的数据仅在尾部空格个数不同，有可能会报duplicate-key错误：

mysql> select c1,c4,c5,hex(c4),hex(c5) from t3;
+------+------+------+----------+----------+
| c1   | c4   | c5   | hex(c4)  | hex(c5)  |
+------+------+------+----------+----------+
|    1 | a    | a    | 61       | 61       |
|    2 | a    | a    | 6120     | 6120     |
|    3 | a    | a    | 612020   | 612020   |
|    4 | a    | a    | 61202020 | 61202020 |
+------+------+------+----------+----------+
4 rows in set (0.00 sec)

mysql> alter table t3 add unique(c4);
ERROR 1062 (23000): Duplicate entry 'a' for key 't3.c4'

mysql> alter table t3 add unique(c5);
Query OK, 0 rows affected (0.44 sec)
Records: 0  Duplicates: 0  Warnings: 0

可以看到c4列创建唯一索引失败，c5列创建唯一索引成功。

c4 utf8mb4_unicode_ci->PAD SPACE->排序和比较运算，忽略字符串尾部空格，4行数据重复。

c5 utf8mb4_0900_ai_ci->NO PAD->排序和比较运算，字符串尾部空格当成普通字符，4行数据不同。

5、总结

Stored

-	CHAR(N)	VARCHAR(N)
Stored	字符不足Ｎ右边补空格	保留插入时的空格，不会在右边额外补充空格

Retrieved

SQL_MODE	CHAR(N)	VARCHAR(N)
Default Value	去掉字符串尾部的空格	保留插入时的空格
PAD_CHAR_TO_FULL_LENGTH	返回完整字符串，不足Ｎ右边补空格	保留插入时的空格

Comparison(不包括like)

| Pad Attribute | CHAR(N)/VARCHAR(N) | | --- | --- |--- | | PAD SPACE | 忽略字符串尾部空格 | | NO PAD | 字符串尾部空格当成普通字符，不能忽略 |

Enjoy GreatSQL :)

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