Tag search

Find posts with a tag using EXISTS.

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.

Find posts tagged 'SQL' using EXISTS.

Show example solution

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

Add EXISTS

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

Show answer

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

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