--- title: SQL Has Rules You Don’t Notice Until Your Query Is Wrong description: A comprehensive guide to SQL behaviors that silently produce wrong results. Covers 20 common pitfalls including NULL three-valued logic, COUNT(*) vs... canonical: https://daily.dev/posts/sql-has-rules-you-don-t-notice-until-your-query-is-wrong-ue7ucmkyu twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: SQL Has Rules You Don’t Notice Until Your Query Is Wrong | daily.dev og:description: A comprehensive guide to SQL behaviors that silently produce wrong results. Covers 20 common pitfalls including NULL three-valued logic, COUNT(*) vs... og:url: https://daily.dev/posts/sql-has-rules-you-don-t-notice-until-your-query-is-wrong-ue7ucmkyu og:image: https://api.daily.dev/og/posts/Ue7UcMKYu.png og:image:alt: SQL Has Rules You Don’t Notice Until Your Query Is Wrong og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # SQL Has Rules You Don’t Notice Until Your Query Is Wrong **[Medium](https://daily.dev/sources/medium_js)** · 12 min read · 0 upvotes · 0 comments ## Summary A comprehensive guide to SQL behaviors that silently produce wrong results. Covers 20 common pitfalls including NULL three-valued logic, COUNT(*) vs COUNT(column) differences, how LEFT JOINs accidentally become INNER JOINs when WHERE filters NULL rows, JOIN row multiplication and data grain, NOT IN with NULLs, WHERE vs HAVING, GROUP BY vs window functions, ROW_NUMBER/RANK/DENSE_RANK differences, date/timestamp filtering, COALESCE and NULLIF usage, SQL execution order, and a 15-point mental checklist for validating queries before trusting results. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Similar posts on daily.dev - [10 Rare SQL Features Every Developer Should Know](https://daily.dev/posts/10-rare-sql-features-every-developer-should-know-fvu4qwczx) · Medium · 0 upvotes · 0 comments --- Tags: [#sql](https://daily.dev/tags/sql) [View this post on daily.dev](https://daily.dev/posts/sql-has-rules-you-don-t-notice-until-your-query-is-wrong-ue7ucmkyu) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"SQL Has Rules You Don’t Notice Until Your Query Is Wrong","url":"https://daily.dev/posts/sql-has-rules-you-don-t-notice-until-your-query-is-wrong-ue7ucmkyu","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/sql-has-rules-you-don-t-notice-until-your-query-is-wrong-ue7ucmkyu"},"datePublished":"2026-07-23T05:35:45.096Z","dateModified":"2026-07-23T05:36:10.601Z","description":"A comprehensive guide to SQL behaviors that silently produce wrong results. Covers 20 common pitfalls including NULL three-valued logic, COUNT(*) vs...","image":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/c121e12353928e06dcf339b60fa49370?_a=AQAEuop","thumbnailUrl":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/c121e12353928e06dcf339b60fa49370?_a=AQAEuop","isAccessibleForFree":true,"articleSection":"Medium","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Medium","logo":"https://media.daily.dev/image/upload/t_logo,f_auto/v1/logos/medium","url":"https://daily.dev/sources/medium_js"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/sql-has-rules-you-don-t-notice-until-your-query-is-wrong-ue7ucmkyu","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"sql","timeRequired":"PT12M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Medium","item":"https://daily.dev/sources/medium_js"},{"@type":"ListItem","position":3,"name":"SQL Has Rules You Don’t Notice Until Your Query Is Wrong"}]} ```