What is the difference between utf8 and utf8mb4 in MySQL, and why does it matter?

Summary

MySQL's old utf8 (an alias of utf8mb3) stores at most 3 bytes per character, so it cannot store emoji or some CJK characters; inserts fail or are truncated. utf8mb4 is real UTF-8 and is the default in MySQL 8.0, with the collation utf8mb4_0900_ai_ci.

Longer Answer

Use utf8mb4 for everything, including the connection character set (SET NAMES utf8mb4 or the driver option); a mismatch between connection and column charset leads to mojibake. Collations decide comparison and sorting: _ai_ci is accent- and case-insensitive (so 'resume' = 'résumé'), _as_cs is sensitive, _bin compares bytes. Unique indexes follow the collation, so case-insensitive collations make [email protected] and [email protected] duplicates. Joining columns with different collations can prevent index use.

Key Points

• utf8 = 3-byte utf8mb3; use utf8mb4.
• Default in 8.0: utf8mb4_0900_ai_ci.
• Collation affects equality, sorting and uniqueness.
• Match connection charset.