- 01Getting Started
- 02SELECT Statement Basics
- 03Filtering Rows with WHERE
- 04Sorting Results with ORDER BY
- 05Combining Tables with JOIN
- 06Aggregating Data with GROUP BY
- 07Fuzzy Matching with LIKE
- 08Modifying Data with INSERT, UPDATE, and DELETE
- 09Introduction to Subqueries
- 10Creating a VIEW
- 11Ranking Rows with Window Functions
- 12Understanding Transactions
- 13Writing Comments in SQL
- 14Logical Operators: AND, OR, NOT
- 15Working with NULL Values
- 16INNER JOIN vs. LEFT JOIN
- 17Filtering Aggregates with HAVING
- 18Removing Duplicates with DISTINCT
- 19Combining Results with UNION
- 20Conditional Values with CASE
- 21Checking Existence with EXISTS
- 22Changing Table Structure with ALTER TABLE
- 23Self Joins
- 24Speeding Up Searches with INDEX
- 25GROUP BY with Multiple Columns
- 26Pagination with LIMIT and OFFSET
- 27Pattern Matching with GLOB
- 28Date Calculations with the date() Function
- 29Joining Three Tables
- 30Matching Multiple Values with IN
- 31Derived Tables: Subqueries in FROM
- 32The Basics of TRIGGER
- 33Readable Queries with WITH (CTEs)
- 34UPSERT: Insert or Update in One Statement
- 35Combining MIN, MAX, and AVG
- 36Restricting Input with CHECK Constraints
- 37Auto-Numbering with AUTOINCREMENT
- 38Inspecting Query Plans with EXPLAIN QUERY PLAN
- 39ORDER BY with Multiple Columns
- 40Conditional Aggregation with CASE
- 41Removing Objects with DROP TABLE and DROP VIEW
- 42Combining Multiple CTEs
- 43Date Range Search with BETWEEN
- 44Finding What's Missing with NOT EXISTS
- 45Substituting NULL with COALESCE
- 46String Functions: UPPER, LENGTH, and SUBSTR
- 47Converting Types with CAST
- 48Merging Grouped Values with GROUP_CONCAT
- 49Enforcing Integrity with FOREIGN KEY
- 50Capstone: Build a Mini Library Management System
Checking Existence with EXISTS
This lesson covers checking whether related data exists using EXISTS, so you can filter efficiently based on presence alone. It's for anyone searching "SQL EXISTS tutorial."
EXISTS(subquery) evaluates to true as soon as the subquery returns at least one row — perfect for "only books that have at least one loan record," where you care about presence, not the actual value.
The example uses WHERE EXISTS (SELECT 1 FROM loans l WHERE l.book_id = b.id) to find books with at least one loan. The 1 inside SELECT 1 has no special meaning — it's just a conventional way to say "I only care whether this returns anything."
A common early mistake is not knowing when to reach for EXISTS over IN — they can produce similar results, but EXISTS only checks "does at least one row exist," rather than evaluating the subquery's actual values, which often makes it faster on large datasets. Deciding behavior based on "does a related record exist" comes up constantly in inventory and lending systems, so mastering EXISTS makes these existence checks much more efficient.
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