SQL

Goal: correct, readable SQL that the planner can execute efficiently.

Use for:

• writing queries, joins, and aggregations
• reviewing query correctness and parameter safety
• replacing row-by-row logic with set-based queries

Workflow:

1. Express intent as a set operation, not a loop.
2. Parameterize all values; never concatenate input.
3. Select only the columns you need.
4. Join on indexed keys; understand the join type you want.
5. Filter early; push predicates into subqueries/CTEs sensibly.
6. Verify correctness on edge cases and read the query plan.

Patterns:

• explicit JOIN ... ON over comma joins
• CTEs for readability of multi-step queries
• window functions for ranking and running totals
• GROUP BY with the right aggregates and HAVING

Rules:

• always parameterize; SQL injection is unacceptable
• avoid SELECT *; name your columns
• know whether NULLs change your results
• check the plan before optimizing by guess