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What is a CTE?

Write a named subquery with WITH.

What is a CTE?

A CTE (Common Table Expression) is a temporary result set. Defined with WITH.

WITH active AS (
  SELECT * FROM customers WHERE status = 'active'
)
SELECT * FROM active;

This query makes active customers a temporary table.

WITH defines a temporary result set (CTE) named active; the query after it uses it like a table. All columns are selected from active.

What is a CTE? — details

A CTE makes a query readable. It replaces subqueries and can be referenced multiple times.

For example, if you need the same active-customer list in both an orders and an invoices report, you define it once as a CTE and refer to it by name twice.

Practice What is a CTE?

Define a CTE named active and SELECT from it.

Starter code

query.sql
-- write CTE
WITH

Sample solution

query.sql
WITH active AS (
  SELECT * FROM customers WHERE status = 'active'
)
SELECT * FROM active;

Expected output

id  name   status
1   Ayşe   active

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Which keyword starts a CTE?

  • WITH Correct answer
  • AS
  • FROM
  • SELECT

Explanation: A CTE starts with WITH.

2 Add WITH

Question 
____ active AS (SELECT * FROM customers WHERE status = 'active')
SELECT * FROM active;
Answer 
WITH active AS (SELECT * FROM customers WHERE status = 'active')
SELECT * FROM active;

Explanation: A CTE definition starts with the WITH keyword, followed by the CTE name and AS.

3 What is a CTE: match the terms

Term Meaning
CTE temporary table
WITH definition
AS name
readable clean

All lessons in Advanced

1 CTE

  1. What is a CTE?
  2. Using WITH
  3. Multiple CTEs
  4. CTE vs subquery
  5. CTE with JOIN
  6. Summary - CTE

2 Recursive CTE

  1. What is a recursive CTE?
  2. Anchor and recursive parts
  3. Walk a hierarchy
  4. Tracking depth
  5. Stopping recursion
  6. Summary - Recursive CTE

3 Window functions

  1. What is a window function?
  2. ROW_NUMBER
  3. RANK and DENSE_RANK
  4. LAG and LEAD
  5. PARTITION BY
  6. Summary - Window functions

4 Project: Analytics report

  1. Analytics design
  2. CTE summary
  3. Top users with ROW_NUMBER
  4. Growth with LAG
  5. Final report

5 Pivot and unpivot

  1. What is pivot?
  2. Pivot with CASE
  3. FILTER clause
  4. Unpivot
  5. Dynamic pivot
  6. Summary - Pivot

6 Materialized view

  1. What is a materialized view?
  2. CREATE
  3. REFRESH
  4. View vs materialized view
  5. Index on materialized views
  6. Summary - Materialized view

7 Stored procedure

  1. What is a procedure?
  2. CREATE PROCEDURE
  3. Parameters
  4. CALL
  5. Control flow
  6. Summary - Stored procedure

8 Function

  1. What is a function?
  2. CREATE FUNCTION
  3. RETURN
  4. Calling a function
  5. Function vs procedure
  6. Summary - Function

9 Trigger

  1. What is a trigger?
  2. BEFORE and AFTER
  3. INSERT trigger
  4. UPDATE trigger
  5. Write an audit trigger
  6. Summary - Trigger

10 Transaction isolation

  1. What is isolation?
  2. READ UNCOMMITTED
  3. READ COMMITTED
  4. REPEATABLE READ
  5. SERIALIZABLE
  6. Summary - Isolation

11 Locking

  1. What is a lock?
  2. Shared and exclusive locks
  3. Deadlock
  4. SELECT FOR UPDATE
  5. Lock timeout
  6. Summary - Locking

12 Query optimization

  1. What is EXPLAIN?
  2. Seq scan vs index scan
  3. Cost
  4. Sargable
  5. N+1 problem
  6. Summary - Optimization

13 Partitioning

  1. What is partitioning?
  2. Range partition
  3. List partition
  4. Hash partition
  5. Partition pruning
  6. Summary - Partitioning

14 Advanced final

  1. Hierarchy report
  2. Window analysis
  3. Procedure and trigger
  4. Optimization