Correlated subqueries

Concepts: sqlSubqueryCorrelated

The defining characteristic is the column reference that crosses query boundaries. The inner query references a column from the outer table alias, creating a dependency: the database cannot execute the subquery independently because it needs context from each outer row. Think of it as a nested loop in code, where the inner loop body runs once per iteration of the outer loop. Understanding Correlated Subqueries The two examples below show how the inner query can reference the outer row: first to pull a per-group aggregate into each detail row, then to trace the execution pass-by-pass. Basic Example This query demonstrates the key pattern: the inner query references a column from the outer query, creating the correlation. How It Executes If you have 3 devices and 6 log entries, the database