Query Structure: Intermediate

CTEs: subqueries are a cry for help

CTEs: subqueries are a cry for help

Category

SQL

Difficulty

intermediate

Duration

28 minutes

Challenges

0 hands-on challenges

Topics covered: CTEs (WITH clause), Query Execution Order, Subqueries for temp results, UNION and UNION ALL, ORDER BY and LIMIT

Lesson Sections

1. CTEs (WITH clause) (concepts: sqlCte)

   A CTE is a named result set you define once at the top of a query, then reference by name anywhere below it. This works exactly the way you assign an intermediate variable in code before using it in a calculation. The filter logic lives inside the CTE, the main query reads from the named result, and the two steps stay cleanly separated. If you have ever computed something once to avoid repeating the same expression twice, you already understand what a CTE does. CTE Usage Examples The following e

2. Query Execution Order (concepts: sqlExecutionOrder)

   Understanding Execution Order Walk through this example to see how the CTE executes before the outer query, not after it. Basic Example This example shows how a CTE executes first to filter logs, then the outer query applies an additional filter on the result. The CTE executes first to produce the temporary dataset. The outer query then filters from it. Practical Implications Understanding execution order helps you avoid common pitfalls like referencing aliases before they exist. Real-World Exam

3. Subqueries for temp results (concepts: sqlSubqueryScalar)

   Subquery Types and Patterns Table Subquery Example Scalar Subquery The subquery retrieves a single value. Each row in the outer query shows the same default name. Multi-row Subquery The subquery lists all active user IDs. The outer query selects orders linked to those users. Practice: Subquery Filter Real-World Example Suppose you need to monitor which devices have many error events. First, count the number of errors per device, then select devices exceeding a certain threshold. Subqueries allow

4. UNION and UNION ALL (concepts: sqlUnion)

   UNION Comparison and Examples Basic Example UNION (aka UNION DISTINCT) UNION ALL Duplicates are preserved. Faster because no deduplication is done. Arrange the UNION Query Practical Usage Real-World Example The performance difference between these two variants is dramatic at scale.

5. ORDER BY and LIMIT (concepts: sqlOrderBy, sqlLimit)

   Control the order and quantity of rows returned with these essential clauses that shape your final output. This returns users sorted by signup date, with the most recent signups appearing first. Restricting Row Counts Check Your Understanding