Aggregation

Aggregate functions collapse many rows into a single number. "How many orders?" "What's the total revenue?" "What's the average basket size?...

An aggregate without GROUP BY collapses the whole table into one row. GROUP BY splits the table into buckets first, then runs the aggregate ...

WHERE filters rows before aggregation. HAVING filters groups after aggregation. That is the entire distinction, and also why HAVING exists a...

CASE is SQL's if/else. It's an expression, not a statement. It produces one value per row, so it can go anywhere a column can: in SELECT, in...