67 lines
2.3 KiB
Markdown
67 lines
2.3 KiB
Markdown
---
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title: Character Sets and Collations
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description: Charset config guide
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tags: mysql, character-sets, utf8mb4, collation, encoding
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---
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# Character Sets and Collations
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## Always Use utf8mb4
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MySQL's `utf8` = `utf8mb3` (3-byte only, no emoji/many CJK). Always `utf8mb4`.
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```sql
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CREATE DATABASE myapp DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci;
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```
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## Collation Quick Reference
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| Collation | Behavior | Use for |
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|---|---|---|
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| `utf8mb4_0900_ai_ci` | Case-insensitive, accent-insensitive | Default |
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| `utf8mb4_0900_as_cs` | Case/accent sensitive | Exact matching |
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| `utf8mb4_bin` | Byte-by-byte comparison | Tokens, hashes |
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`_0900_` = Unicode 9.0 (preferred over older `_unicode_` variants).
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## Collation Behavior
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Collations affect string comparisons, sorting (`ORDER BY`), and pattern matching (`LIKE`):
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- **Case-insensitive (`_ci`)**: `'A' = 'a'` evaluates to true, `LIKE 'a%'` matches 'Apple'
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- **Case-sensitive (`_cs`)**: `'A' = 'a'` evaluates to false, `LIKE 'a%'` matches only lowercase
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- **Accent-insensitive (`_ai`)**: `'e' = 'é'` evaluates to true
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- **Accent-sensitive (`_as`)**: `'e' = 'é'` evaluates to false
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- **Binary (`_bin`)**: strict byte-by-byte comparison (most restrictive)
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You can override collation per query:
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```sql
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SELECT * FROM users
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WHERE name COLLATE utf8mb4_0900_as_cs = 'José';
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```
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## Migrating from utf8/utf8mb3
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```sql
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-- Find columns still using utf8
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SELECT table_name, column_name FROM information_schema.columns
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WHERE table_schema = 'mydb' AND character_set_name = 'utf8';
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-- Convert
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ALTER TABLE users CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci;
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```
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**Warning**: index key length limits depend on InnoDB row format:
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- DYNAMIC/COMPRESSED: 3072 bytes max (≈768 chars with utf8mb4)
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- REDUNDANT/COMPACT: 767 bytes max (≈191 chars with utf8mb4)
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`VARCHAR(255)` with utf8mb4 = up to 1020 bytes (4×255). That's safe for DYNAMIC/COMPRESSED but exceeds REDUNDANT/COMPACT limits.
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## Connection
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Ensure client uses `utf8mb4`: `SET NAMES utf8mb4;` (most modern drivers default to this).
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`SET NAMES utf8mb4` sets three session variables:
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- `character_set_client` (encoding for statements sent to server)
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- `character_set_connection` (encoding for statement processing)
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- `character_set_results` (encoding for results sent to client)
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It also sets `collation_connection` to the default collation for utf8mb4.
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