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Relational Foundations
SELECT literals Column aliases DISTINCT rows ORDER BY LIMIT and OFFSET NULL semantics Three-valued logic CASE expressions COALESCE NULLIF
Filtering
WHERE predicates Comparison operators BETWEEN IN lists LIKE patterns ESCAPE in LIKE IS NULL Boolean combinations Operator precedence EXISTS
Aggregation
COUNT SUM AVG MIN and MAX GROUP BY HAVING Conditional aggregation Grouping expressions COUNT DISTINCT Aggregate NULL behavior
Joins
INNER JOIN LEFT JOIN RIGHT JOIN FULL OUTER JOIN CROSS JOIN Self join Non-equi join Composite join keys Join filtering Anti join with NOT EXISTS
Subqueries & Sets
Scalar subquery Correlated subquery Derived table CTE Recursive CTE UNION UNION ALL INTERSECT EXCEPT Subquery with ANY Subquery with ALL
Window Functions
ROW_NUMBER RANK DENSE_RANK PARTITION BY Window ORDER BY ROWS frame RANGE frame LAG LEAD Running total
Data Changes
INSERT Multi-row INSERT INSERT SELECT UPDATE UPDATE with subquery DELETE MERGE Transactions SAVEPOINT ROLLBACK
Schema & Integrity
CREATE TABLE PRIMARY KEY FOREIGN KEY UNIQUE constraint CHECK constraint DEFAULT values ALTER TABLE CREATE INDEX Composite index VIEW
SQL / Aggregation

COUNT DISTINCT

Learn COUNT DISTINCT in SQL through a focused explanation, a dedicated construct-specific example, an engineering boundary, and a mastery check.

10 min read • Beginner

COUNT DISTINCT is an atomic SQL topic inside Aggregation. This lesson focuses on how the COUNT DISTINCT construct is written, what it changes, and when it is the right tool.

Trace the example line by line. Identify the exact syntax for COUNT DISTINCT, the input state before it runs, and the resulting value, control flow, database state, hardware behavior, or runtime effect afterward. Neighboring lessons use different constructs rather than renamed copies of this one.

Topic-specific example

sql-03-09-count-distinct.sql
sql Try It Live 
SELECT COUNT(DISTINCT user_id) AS active_users FROM sessions;
Edge case / engineering note

For COUNT DISTINCT, test the boundary that belongs to this exact feature rather than a generic failure case. Check invalid or empty input, NULL/unknown state, numeric limits, transaction or exception behavior, synthesis/runtime constraints, ownership/permission rules, or backtracking/control-flow effects where they apply.

Content source & attribution
Original Syntaxis Content · Original Syntaxis Content

Lesson quizzes

COUNT DISTINCT Mastery Check →
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