Chris Perry’s Post

SQL WHERE clause: Ditch CASE, embrace Boolean logic There's a pattern I see in almost every SQL codebase I review: CASE expressions buried inside WHERE clauses. It looks logical. It compiles fine. But it silently destroys query performance — because the moment you wrap a column in CASE, the optimizer can no longer use the index. Every row gets evaluated. Every time. The fix is one line: replace CASE with plain Boolean logic — AND, OR, NOT. That's it. Boolean predicates are SARGable, meaning the engine seeks directly to matching rows and skips everything else. On a 1M row table, that's the difference between 3,200 ms and 52 ms. Same data. Same indexes. Zero schema changes. Full breakdown, real benchmark data, and a pattern cheat sheet below. 👇 https://lnkd.in/dJJyUttS #SQL #DataEngineering #QueryOptimization #SQLServer #DatabasePerformance

🚀 JOIN vs Subquery – Which is Faster? Still confused between JOIN and Subquery in SQL? 👉 Here’s the simple logic: ✔ Subquery = Query inside query ✔ JOIN = Combine tables ✔ JOIN works faster for large data ⚡ ✔ Execution plan decides performance 💡 Learn smart, write optimized queries! 💬 Comment: JOIN or Subquery? 📌 Follow for more SQL tips

🚀 EXISTS in SQL 💡 What is EXISTS? ✔️ Check if a subquery returns any rows ✔️ Returns TRUE if at least 1 row exists ✔️ Returns FALSE if no rows 🔍 How it works: ➡️ Outer query runs ➡️ Inner query checks for matching rows ➡️ If a match is found, the row is included ➡️ If no match, the row is excluded 🗝️ Key Concept 👉 EXISTS does not care about values 👉 It only checks if rows exist 🧠 Even if the subquery returns NULL ➡️ EXISTS can still be TRUE #SQL #DataAnalytics #LearningSQL #TechTips #InterviewPrep #DataEngineering

The five stages of finding someone else's SQL: 1. This looks straightforward. 2. Wait, why is there a subquery inside a subquery inside a subquery? 3. What does this column name even mean? 4. Who hurt you? 5. I'll just rewrite the whole thing and pretend I never saw this. #datahumor

Day 26/90 — SQL Series | Week 4 "My WHERE clause is not filtering dates correctly." "My SUM is returning NULL instead of a number." 9 times out of 10 — it's a data type mismatch. CAST and CONVERT fix it. CAST('250' AS INT) → turns text into number so you can do math CAST('2024-01-15' AS DATE) → turns text into date so filters work CAST(order_id AS VARCHAR) → turns number into text for concatenation Rule: use CAST (works everywhere). Use CONVERT only when you need date formatting in SQL Server. #SQL #CastConvert #DataAnalytics #LearnSQL #DataAnalyst

i thought SQL queries execute step by step but but that’s not how it actually works SQL doesn’t start with SELECT it starts with FROM here is the logical order SQL processes a query : 👇 1️⃣ SQL gets the data (FROM) 2️⃣ combines tables if needed (JOIN/ON) 3️⃣ filters rows (WHERE) 4️⃣ groups data (GROUP BY) 5️⃣ filters groups (HAVING) 6️⃣ selects columns (SELECT) 7️⃣ removes duplicates (DISTINCT) 8️⃣ sorts results (ORDER BY) 9️⃣ limits the output (LIMIT) once I understood this, writing queries felt a lot easier 😁 #dataanalyst #dataanalytics #sql #learninginpublic

SQL query optimization is 90% understanding your data model and 10% memorizing syntax. I spent 3 years writing slow queries because nobody taught me to check JOIN cardinality first. Now every query I write starts with: "How many rows will this return?"

Every day, I see posts claiming the "SQL execution order" is incorrect because FROM comes after SELECT. Who will tell them that their recursive SQL is an iterative execution? 🤣 https://lnkd.in/e8jtP7qW