Basic Querying & Filtering
Master SELECT, WHERE, ORDER BY, LIMIT, DISTINCT, BETWEEN, LIKE, IN, IS NULL, and basic column aliasing.
SQL Query Architecture & Optimization Guide
Basic querying encompasses sorting, limiting, deduplicating, and matching patterns across relational records using ORDER BY, LIMIT, OFFSET, DISTINCT, and pattern operators like LIKE and ILIKE.
Ordering and pagination occur at the very end of query execution: FROM -> WHERE -> SELECT -> DISTINCT -> ORDER BY -> LIMIT. The database engine sorts the projected result set using quicksort or external merge sort before applying row offsets.
Using high OFFSET numbers (such as OFFSET 50000) forces the engine to read and discard 50,000 rows. Use keyset pagination (WHERE id > last_seen_id) for high performance. Also, remember that AND has higher operator precedence than OR; always use parentheses.
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Frequently Asked Questions on Basic Querying & Filtering
Why is OFFSET pagination slow for large database tables?
OFFSET requires the database engine to scan, sort, and discard all preceding rows before returning the requested page. Keyset pagination using indexed WHERE conditions avoids scanning discarded records completely.
How does DISTINCT affect query performance?
DISTINCT requires sorting or hashing the entire intermediate result set to identify and eliminate duplicate rows, consuming additional CPU and temporary memory buffer space.
What is the order of precedence between AND and OR in SQL predicates?
AND takes precedence over OR. When combining both operators in a single WHERE clause, wrap OR conditions in parentheses to ensure correct logical grouping.