SQL · Pattern 11 — Interview SQL Patterns
Conditional Aggregation
Uses CASE inside aggregate functions.
Source data — Pattern 11: Interview SQL Patterns
All queries run against the shared employees table (see schema.sql). Topics 71 and 72 introduce an orders table to solve consecutive records and gaps-and-islands problems. Topics 73 and 74 introduce a monthly_sales table to demonstrate PIVOT and UNPIVOT operations.
What it does
Uses CASE inside aggregate functions.
Problem
Count employees by salary category.
SELECT
COUNT(CASE WHEN salary<50000 THEN 1 END) AS low_salary,
COUNT(CASE WHEN salary BETWEEN 50000 AND 80000 THEN 1 END) AS medium_salary,
COUNT(CASE WHEN salary>80000 THEN 1 END) AS high_salary
FROM employees;
Result
