การรวมแบบมีเงื่อนไขด้วย Case/When

Sometimes you need different results for different rows. Case lets the database choose a value per row, like an if statement in SQL.

A Case holds one or more When branches; each When pairs a condition with the value to use when that condition is true.

Annotate a computed label by giving each When a condition and a then value, with a default for the rest.

The real power comes from putting Case inside an aggregate, so you can count or sum only the rows that match a condition.

Wrap a Case in Count to tally just the matching rows, since non-matching branches return null and are skipped.

Modern Django offers a cleaner way: pass filter to an aggregate to count or sum a subset without writing Case yourself.

Annotate several conditional aggregates together to build a breakdown, like active and inactive counts in one query.

The same trick works with Sum, letting you total only the rows that pass a condition, such as paid invoices.

When the value types are not obvious, give Case an output_field so Django knows how to handle the result.

You can even order_by a Case annotation to create custom sort priorities that plain field ordering cannot express.

All of this branching runs server-side in a single query, far faster than looping in Python and counting by hand.

You want one number: how many members of a team are active, computed in the database.

Case and When add per-row branching, and pairing conditions with aggregates, or the filter= shortcut, builds rich breakdowns in one query. 📊