SQL Dialects
SQL Dialects
Definition
A SQL dialect is a specific database vendor's implementation of SQL — the standard core plus that vendor's own proprietary syntax, functions, and behavior. MySQL, PostgreSQL, SQL Server (T-SQL), Oracle (PL/SQL), and SQLite each speak a different dialect.
How It Works — Same Idea, Different Spelling
The clearest place dialects diverge is auto-incrementing primary keys and row-limiting:
| Task | MySQL | PostgreSQL | SQL Server | SQLite | ||||
|---|---|---|---|---|---|---|---|---|
| Auto-increment | AUTO_INCREMENT | SERIAL / GENERATED ALWAYS AS IDENTITY | IDENTITY(1,1) | INTEGER PRIMARY KEY (auto-rowid) | ||||
| Limit rows | LIMIT 10 | LIMIT 10 | TOP 10 | LIMIT 10 | ||||
| String concatenation | CONCAT(a,b) | `a \ | \ | b` | a + b | `a \ | \ | b` |
A query that runs perfectly on MySQL can fail outright on SQL Server purely because of these spelling differences — the underlying idea (auto-increment a key, limit a result set) is identical everywhere.
Edge Cases and Pitfalls
- Copy-pasting SQL from a tutorial or Stack Overflow answer without checking which dialect it targets is one of the most common sources of "why doesn't this work" bugs for students.
- Some dialect differences are silent, not error-producing: integer division truncates in some engines but returns a decimal in others (this repo's own SQL judge design had to account for exactly this — MySQL keeps the fractional part on
/, whileDIVtruncates). - Date/time functions are especially dialect-specific (
NOW()vsGETDATE()vsCURRENT_TIMESTAMP), even thoughCURRENT_TIMESTAMPitself is standard and widely supported.
Key Takeaways
- A dialect = standard SQL + one vendor's own extensions and quirks.
- The same task is often expressed with different keywords/functions across dialects — learn the underlying concept, then look up the dialect-specific spelling.
- When following any SQL tutorial or example, always note which database it targets before assuming it will run unchanged elsewhere.