Add calculated columns using subqueries in SELECT clause

Master scalar subqueries that return same value for all rows

Use correlated subqueries that calculate different values per row

Calculate per-row aggregates: order count, total spent, department average

Understand performance impact: scalar (fast) vs correlated (slow)

Choose between subqueries, JOINs, and window functions for best performance

Optimize correlated subqueries with proper indexes

Handle NULL results and multiple subqueries in SELECT

Add calculated columns using subqueries in SELECT clause

Master scalar subqueries that return same value for all rows

Use correlated subqueries that calculate different values per row

Calculate per-row aggregates: order count, total spent, department average

Understand performance impact: scalar (fast) vs correlated (slow)

Choose between subqueries, JOINs, and window functions for best performance

Optimize correlated subqueries with proper indexes

Handle NULL results and multiple subqueries in SELECT