How do aggregate functions handle NULL?

All standard aggregates except COUNT(*) ignore NULL inputs: AVG, SUM, MIN, MAX and COUNT(col) skip NULL rows. COUNT(*) counts every row including those with NULLs. If every input is NULL, AVG/SUM/MIN/MAX return NULL rather than 0.

What is the difference between COUNT(*) and COUNT(column)?

COUNT(*) counts all rows in the group regardless of content. COUNT(column) counts only rows where that column is not NULL. COUNT(DISTINCT column) counts the number of distinct non-NULL values, which is often what you want for cardinality.

How do I concatenate grouped strings in each database?

MySQL and SQLite use GROUP_CONCAT(expr SEPARATOR ','). PostgreSQL and SQL Server use STRING_AGG(expr, ','). Oracle uses LISTAGG. They differ in separator syntax and ordering clauses, so this is a common portability snag.

Can I combine DISTINCT with an aggregate?

Yes. SUM(DISTINCT col), COUNT(DISTINCT col) and AVG(DISTINCT col) aggregate only over unique values. DISTINCT applies before the aggregate runs. Note that GROUP_CONCAT(DISTINCT ...) is supported in MySQL and SQLite but STRING_AGG handles uniqueness differently.

Does this tool send my query anywhere?

No. The function list is bundled into the page and filtered entirely in your browser. Nothing is uploaded, logged or stored.

SQL Aggregate Functions Reference

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What aggregate functions do

An aggregate function collapses many rows into a single value per group — a sum, an average, a row count, a concatenated string. They pair with GROUP BY to produce one result row per group, or run over the whole table with no GROUP BY. This reference lists the common aggregates, their NULL behaviour, and which of MySQL, PostgreSQL, SQLite and SQL Server support each name.

How it works

Each aggregate scans the rows of a group and folds them into one value, ignoring NULL inputs (except COUNT(*)):

SELECT department,
       COUNT(*)              AS headcount,
       AVG(salary)           AS avg_salary,
       SUM(DISTINCT bonus)   AS unique_bonus_total,
       STRING_AGG(name, ', ') AS roster
FROM   employees
GROUP  BY department;

DISTINCT inside an aggregate restricts it to unique values. String aggregation is the least portable area: MySQL/SQLite spell it GROUP_CONCAT while PostgreSQL/SQL Server use STRING_AGG. The tool filters the table as you type and flags which databases provide each function.

Tips and notes

COUNT(*) never returns NULL; the all-NULL group still counts its rows.
AVG of an empty or all-NULL set is NULL, not 0 — wrap in COALESCE if needed.
Mixing aggregated and non-aggregated columns requires every plain column in GROUP BY.
FILTER (WHERE ...) (Postgres/SQLite) is a cleaner alternative to SUM(CASE WHEN ...).