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[手动选题][tech]: 20230110.0 ⭐️⭐️ A guide to strings in MySQL.md

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[#]: subject: "A guide to strings in MySQL"
[#]: via: "https://opensource.com/article/23/1/strings-mysql"
[#]: author: "Hunter Coleman https://opensource.com/users/hunterc"
[#]: collector: "lkxed"
[#]: translator: " "
[#]: reviewer: " "
[#]: publisher: " "
[#]: url: " "
A guide to strings in MySQL
======
Strings are one of the most common data types you will use in MySQL. Many users insert and read strings in their databases without thinking too much about them. This article aims to give you a bit of a deep dive into how MySQL stores and displays your string variables so that you can have better control over your data.
You can break strings into two categories: binary and nonbinary. You probably think about nonbinary strings most of the time. Nonbinary strings have character sets and collations. Binary strings, on the other hand, store things such as MP3 files or images. Even if you store a word in a binary string, such as **song**, it is not stored in the same way as in a nonbinary string.
I will focus on nonbinary strings. All nonbinary strings in MySQL are associated with a character set and a collation. A string's character set controls what characters can be stored in the string, and its collation controls how the strings are ordered when you display them.
### Character sets
To view the character sets on your system, run the following command:
```
SHOW CHARACTER SET;
```
This command will output four columns of data, including the character set:
- Name
- Brief description
- Default collation
- Maximum size of each character in the character set
MySQL used to default to the **latin1** character set, but since version 8.0, the default has been **utf8mb4**. The default collation is now **utf8mb4_0900_ai_ci**. The **ai** indicates that this collation is accent insensitive (**á = a**), and the **ci** specifies that it is case insensitive (**a = A**).
Different character sets store their characters in various-sized chunks of memory. For example, as you can see from the above command, characters stored in **utf8mb4** are stored in memory from one to four bytes in size. If you want to see if a string has multibyte characters, you can use the **CHAR_LENGTH()** and **LENGTH()** functions. **CHAR_LENGTH()** displays how many characters a string contains, whereas **LENGTH()** shows how many bytes a string has, which may or may not be the same as a string's length in characters, depending on the character set. Here is an example:
```
SET @a = CONVERT('data' USING latin1);
SELECT LENGTH(@a), CHAR_LENGTH(@a);
+------------+-----------------+
| LENGTH(@a) | CHAR_LENGTH(@a) |
+------------+-----------------+
| 4 | 4 |
+------------+-----------------+
```
This example shows that the **latin1** character set stores characters in single-byte units. Other character sets, such as **utf16**, allow multibyte characters:
```
SET @b = CONVERT('data' USING utf16);
SELECT LENGTH(@b), CHAR_LENGTH(@b);
+------------+------------------+
| LENGTH(@b) | CHAR_LENGTH(@b) |
+------------+------------------+
| 8 | 4 |
+------------+------------------+
```
### Collation
A string's collation will determine how the values are displayed when you run a SQL statement with an **ORDER BY** clause. Your choice of collations is determined by what character set you select. When you ran the command `SHOW CHARACTER SET` above, you saw the default collations for each character set. You can easily see all the collations available for a particular character set. For example, if you want to see which collations are allowed by the **utf8mb4** character set, run:
```
SHOW COLLATION LIKE 'utf8mb4%';
```
A collation can be case-insensitive, case-sensitive, or binary. Let's build a simple table, insert a few values into it, and then view the data using different collations to see how the output differs:
```
CREATE TABLE sample (s CHAR(5));
INSERT INTO sample (s) VALUES
('AAAAA'), ('ccccc'), ('bbbbb'), ('BBBBB'), ('aaaaa'), ('CCCCC');
SELECT * FROM sample;
+-----------+
| s |
+-----------+
| AAAAA |
| ccccc |
| bbbbb |
| BBBBB |
| aaaaa |
| CCCCC |
+-----------+
```
With case-insensitive collations, your data is returned in alphabetical order, but there is no guarantee that capitalized words will come before lowercase words, as seen below:
```
SELECT * FROM sample ORDER BY s COLLATE utf8mb4_turkish_ci;
+-----------+
| s |
+-----------+
| AAAAA |
| aaaaa |
| bbbbb |
| BBBBB |
| ccccc |
| CCCCC |
+-----------+
```
On the other hand, when MySQL runs a case-sensitive search, lowercase will come before uppercase for each letter:
```
SELECT * FROM sample ORDER BY s COLLATE utf8mb4_0900_as_cs;
+-----------+
| s |
+-----------+
| aaaaa |
| AAAAA |
| bbbbb |
| BBBBB |
| ccccc |
| CCCCC |
+-----------+
```
And binary collations will return all capitalized words before lowercase words:
```
SELECT * FROM sample ORDER BY s COLLATE utf8mb4_0900_bin;
+-----------+
| s |
+-----------+
| AAAAA |
| ccccc |
| bbbbb |
| BBBBB |
| aaaaa |
| CCCCC |
+-----------+
```
If you want to know which character set and collation a string uses, you can use the aptly named **charset** and **collation** functions. A server running MySQL version 8.0 or higher will default to using the **utf8mb4** character set and **utf8mb4_0900_ai-ci** collation:
```
SELECT charset('data');
+-------------------+
| charset('data') |
+-------------------+
| utf8mb4 |
+-------------------+
SELECT collation('data');
+--------------------+
| collation('data') |
+--------------------+
| utf8mb4_0900_ai_ci |
+--------------------+
```
You can use the `SET NAMES` command to change the character set or collation used.
To change from the **utf8mb4** character set to **utf16**, run this command:
```
SET NAMES 'utf16';
```
If you would also like to choose a collation other than the default, you can add a **COLLATE** clause to the `SET NAMES` command.
For example, say your database stores words in the Spanish language. The default collation for MySQL (**utf8mb4_0900_ai_ci**) sees ch and ll as two different characters and will sort them as such. But in Spanish, ch and ll are individual letters, so if you want them sorted in the proper order (following c and l, respectively), you need to use a different collation. One option is to use the **utf8mb4_spanish2_ci** collation.
```
SET NAMES 'utf8mb4' COLLATE 'utf8mb4_spanish2-ci';
```
### Storing strings
MySQL allows you to choose between several data types for your string values. (Even more so than other popular databases such as PostgreSQL and MongoDB.)
Here is a list of MySQL's binary string data types, their nonbinary equivalents, and their maximum length:
- **binary:** char (255)
- **varbinary:** varchar (65,535)
- **tinyblob:** tinytext (255)
- **blob:** text (65,535)
- **mediumblob:** mediumtext (16,777,215)
- **longblob:** longtext (4,294,967,295)
One important thing to remember is that unlike the varbinary, varchar, text, and blob types, which are stored in variable length fields (that is, using only as much space as needed), MySQL stores binary and char types in fixed length fields. So a value such as **char(20)** or **binary(20)** will always take up 20 bytes, even if you store less than 20 characters in them. MySQL pads the values with the **ASCII NUL** value (**0x00**) for binary types and spaces for char types.
Another thing to consider when choosing data types is whether you want spaces after the string to be preserved or stripped. When displaying data, MySQL strips whitespace from data stored with the char data type, but not varchar.
```
CREATE TABLE sample2 (s1 CHAR(10), s2 VARCHAR(10));
INSERT INTO sample2 (s1, s2) VALUES ('cat ', 'cat ');
SELECT s1, s2, CHAR_LENGTH(s1), CHAR_LENGTH(s2) FROM sample2;
+---------+---------+-----------------------------------+
| s1 | s2 | CHAR_LENGTH(s1) | CHAR_LENGTH(s2) |
+---------+---------+-----------------------------------+
| cat | cat | 3 | 10 |
+---------+---------+-----------------------------------+
```
### Wrap up
Strings are one of the most common data types used in databases, and MySQL remains one of the most popular database systems in use today. I hope that you have learned something new from this article and will be able to use your new knowledge to improve your database skills.
--------------------------------------------------------------------------------
via: https://opensource.com/article/23/1/strings-mysql
作者：[Hunter Coleman][a]
选题：[lkxed][b]
译者：[译者ID](https://github.com/译者ID)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]: https://opensource.com/users/hunterc
[b]: https://github.com/lkxed