add brain

.brain/.agent/skills/database-optimization/mysql/references/character-sets.md

@@ -0,0 +1,66 @@
+---
+title: Character Sets and Collations
+description: Charset config guide
+tags: mysql, character-sets, utf8mb4, collation, encoding
+---
+
+# Character Sets and Collations
+
+## Always Use utf8mb4
+MySQL's `utf8` = `utf8mb3` (3-byte only, no emoji/many CJK). Always `utf8mb4`.
+
+```sql
+CREATE DATABASE myapp DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci;
+```
+
+## Collation Quick Reference
+| Collation | Behavior | Use for |
+|---|---|---|
+| `utf8mb4_0900_ai_ci` | Case-insensitive, accent-insensitive | Default |
+| `utf8mb4_0900_as_cs` | Case/accent sensitive | Exact matching |
+| `utf8mb4_bin` | Byte-by-byte comparison | Tokens, hashes |
+
+`_0900_` = Unicode 9.0 (preferred over older `_unicode_` variants).
+
+## Collation Behavior
+
+Collations affect string comparisons, sorting (`ORDER BY`), and pattern matching (`LIKE`):
+
+- **Case-insensitive (`_ci`)**: `'A' = 'a'` evaluates to true, `LIKE 'a%'` matches 'Apple'
+- **Case-sensitive (`_cs`)**: `'A' = 'a'` evaluates to false, `LIKE 'a%'` matches only lowercase
+- **Accent-insensitive (`_ai`)**: `'e' = 'é'` evaluates to true
+- **Accent-sensitive (`_as`)**: `'e' = 'é'` evaluates to false
+- **Binary (`_bin`)**: strict byte-by-byte comparison (most restrictive)
+
+You can override collation per query:
+
+```sql
+SELECT * FROM users
+WHERE name COLLATE utf8mb4_0900_as_cs = 'José';
+```
+
+## Migrating from utf8/utf8mb3
+
+```sql
+-- Find columns still using utf8
+SELECT table_name, column_name FROM information_schema.columns
+WHERE table_schema = 'mydb' AND character_set_name = 'utf8';
+-- Convert
+ALTER TABLE users CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci;
+```
+
+**Warning**: index key length limits depend on InnoDB row format:
+- DYNAMIC/COMPRESSED: 3072 bytes max (≈768 chars with utf8mb4)
+- REDUNDANT/COMPACT: 767 bytes max (≈191 chars with utf8mb4)
+
+`VARCHAR(255)` with utf8mb4 = up to 1020 bytes (4×255). That's safe for DYNAMIC/COMPRESSED but exceeds REDUNDANT/COMPACT limits.
+
+## Connection
+Ensure client uses `utf8mb4`: `SET NAMES utf8mb4;` (most modern drivers default to this).
+
+`SET NAMES utf8mb4` sets three session variables:
+- `character_set_client` (encoding for statements sent to server)
+- `character_set_connection` (encoding for statement processing)
+- `character_set_results` (encoding for results sent to client)
+
+It also sets `collation_connection` to the default collation for utf8mb4.