- 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
Finding What's Missing with NOT EXISTS
This lesson covers finding "missing" data with NOT EXISTS, so you can efficiently locate rows with no related records. It's for anyone searching "SQL NOT EXISTS tutorial."
NOT EXISTS(subquery) evaluates to true when the subquery returns zero rows — useful for something like "books that have never been checked out," where you're looking for the complete absence of related data.
The example uses WHERE NOT EXISTS (SELECT 1 FROM loans l WHERE l.book_id = b.id) to find books with zero loan records — the exact mirror image of the EXISTS pattern from an earlier lesson.
A common early mistake is not knowing when NOT IN and NOT EXISTS diverge — they can look interchangeable, but they behave differently when NULL shows up among the compared values, and NOT EXISTS is generally the safer default in practice. This is the mirror image of EXISTS, purpose-built for efficiently finding "records with absolutely no related data" — a routine data-maintenance task, like flagging inventory items that have never been ordered.
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