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Tag search

Find posts with a tag using EXISTS.

Tag search

To find posts with a specific tag you use EXISTS.

SELECT title FROM posts p
WHERE EXISTS (SELECT 1 FROM post_tags t WHERE t.post_id = p.id AND t.tag = 'SQL');

This query returns posts tagged SQL.

title is selected from posts p. WHERE keeps only the rows where EXISTS (SELECT 1 FROM post_tags t WHERE t.post_id = p.id … holds.

Tag search — details

EXISTS uses a correlated subquery. It stops at the first match, so it is fast.

For example, to list posts tagged "SQL" you check whether a matching row exists in the post_tags table.

Practice Tag search

Find posts tagged 'SQL' using EXISTS.

Starter code

query.sql
-- tagged posts
SELECT title FROM posts p
WHERE

Sample solution

query.sql
SELECT title FROM posts p
WHERE EXISTS (SELECT 1 FROM post_tags t WHERE t.post_id = p.id AND t.tag = 'SQL');

Expected output

title
SQL temelleri
JOIN rehberi

Questions and answers

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

1 What is fast for tag search?

  • DISTINCT
  • COUNT
  • IN
  • EXISTS Correct answer

Explanation: EXISTS stops at the first match, so it is fast.

2 Add EXISTS

Question 
SELECT title FROM posts p
WHERE ____ (SELECT 1 FROM post_tags t WHERE t.post_id = p.id AND t.tag = 'SQL');
Answer 
SELECT title FROM posts p
WHERE EXISTS (SELECT 1 FROM post_tags t WHERE t.post_id = p.id AND t.tag = 'SQL');

Explanation: EXISTS finds posts tagged SQL; the subquery is true once it finds at least one match.

3 Tag search: match the terms

Term Meaning
EXISTS existence
tag tag
correlated outer-dependent
fast short-circuit

All lessons in Intermediate

1 INNER JOIN in depth

  1. INNER JOIN logic
  2. Choosing the join column
  3. Three-table INNER JOIN
  4. Self JOIN
  5. Filtering joins
  6. Summary - INNER JOIN

2 LEFT and RIGHT JOIN

  1. LEFT JOIN
  2. RIGHT JOIN
  3. Unmatched rows
  4. JOINs with NULL
  5. LEFT vs RIGHT
  6. Summary - LEFT and RIGHT JOIN

3 FULL and CROSS JOIN

  1. FULL OUTER JOIN
  2. CROSS JOIN
  3. When to CROSS JOIN
  4. Comma JOIN
  5. Choosing a JOIN type
  6. Summary - FULL and CROSS JOIN

4 Project: Library database

  1. Library design
  2. Books and authors
  3. List loans with LEFT JOIN
  4. Find overdue loans
  5. Member summary

5 GROUP BY

  1. What is GROUP BY?
  2. Single-column grouping
  3. Multi-column grouping
  4. GROUP BY with aggregates
  5. Ordering grouped results
  6. Summary - GROUP BY

6 HAVING

  1. What is HAVING?
  2. HAVING vs WHERE
  3. HAVING with aggregates
  4. Multiple conditions
  5. HAVING with aliases
  6. Summary - HAVING

7 Aggregate functions

  1. COUNT
  2. SUM
  3. AVG
  4. MIN and MAX
  5. NULL and aggregates
  6. Summary - Aggregates

8 Project: Sales report

  1. Sales design
  2. Daily revenue
  3. Best sellers
  4. Customer segments
  5. Low performers

9 Subquery

  1. What is a subquery?
  2. Scalar subquery
  3. Subquery with IN
  4. ANY and ALL
  5. Correlated subquery
  6. Summary - Subquery

10 EXISTS

  1. What is EXISTS?
  2. NOT EXISTS
  3. EXISTS vs IN
  4. EXISTS performance
  5. Multiple EXISTS
  6. Summary - EXISTS

11 UNION and INTERSECT

  1. UNION
  2. UNION ALL
  3. INTERSECT
  4. EXCEPT
  5. Set rules
  6. Summary - UNION and INTERSECT

12 CASE

  1. What is CASE?
  2. Searched CASE
  3. Simple CASE
  4. The ELSE branch
  5. CASE with aggregates
  6. Summary - CASE

13 VIEW

  1. What is a VIEW?
  2. CREATE VIEW
  3. Querying a VIEW
  4. DROP VIEW
  5. Updatable VIEWs
  6. Summary - VIEW

14 INDEX basics

  1. What is an INDEX?
  2. CREATE INDEX
  3. Composite INDEX
  4. DROP INDEX
  5. When to index
  6. Summary - INDEX

15 Project: Blog database

  1. Blog design
  2. Post list
  3. Tag search
  4. Author stats
  5. Search index

16 Normalization

  1. What is normalization?
  2. First normal form
  3. Second normal form
  4. Third normal form
  5. BCNF
  6. Summary - Normalization

17 Data integrity

  1. What is integrity?
  2. Entity integrity
  3. Referential integrity
  4. Domain integrity
  5. Integrity with triggers
  6. Summary - Data integrity

18 Transaction basics

  1. What is a transaction?
  2. BEGIN and COMMIT
  3. ROLLBACK
  4. SAVEPOINT
  5. ACID
  6. Summary - Transaction

19 ALTER and DROP

  1. ALTER TABLE
  2. Adding a column
  3. Dropping a column
  4. Changing a column type
  5. DROP TABLE
  6. Summary - ALTER and DROP

20 Intermediate final

  1. Multi-JOIN report
  2. Aggregate analysis
  3. Subquery challenge
  4. Transaction scenario