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SQL & Databases Masterclass Course

Welcome to the ultimate postgresql tutorial for beginners 2026. Work through interactive sql practice problems with solutions, master our comprehensive sql window functions tutorial modules, and prepare for tough sql interview questions at Google and Amazon entirely in your browser. Claim your course completion XP and build production-ready database mastery today!

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40 Lessons
LESSON WORKSPACEBeginner

SQL Lesson 2: Queries with constraints (Pt. 1)

Introduction & Core Concept

Retrieving all rows from a table is great for exploring, but real-world analytics is all about precision. To fetch only the rows that satisfy specific criteria, we append a WHERE constraint directly after the FROM statement.

Standard mathematical comparison operators are fully supported:

  • = : Strictly equal to
  • != or <> : Not equal to
  • < : Less than
  • > : Greater than
  • <= , >= : Less than or equal to / Greater than or equal to
    • Why & Where We Use It
  • Why We Use It: Filtering data at the database level is thousands of times faster than downloading the entire table into Python or Javascript and filtering it there.
  • Where We Use It: Used whenever you need to find specific user IDs, isolate high-value transactions, or exclude errors and test accounts.
    • Real-World Example

    The inventory manager at ShopMart wants to identify premium products that cost more than $500 so they can be placed in a secure glass display case. They add the filter WHERE price > 500.00.

    Best Practices: What to Do & What NOT to Do
  • What to Do: When combining multiple mathematical conditions using AND and OR, always wrap related clauses in parentheses to guarantee the exact evaluation order you intend.
  • What NOT to Do: Never compare numerical fields against text strings wrapped in quotes (e.g., WHERE price > '500'). This forces the database engine to perform slow, implicit data type conversions.
    • Take Action & Practice
  • Practice: Try these constraints on the live dataset in the [SQL Playground](/playground).
  • Reference: Keep the [SQL Cheat Sheet](/cheatsheet) open in a new tab for a quick syntax reference!
  • Next Lesson: Master text filters in [Queries with Constraints Part 2](/lessons/queries-with-constraints-part-2).
    • Interactive Sandboxed Terminal (Preloaded DB Schema: SHOPMART)
    SQL Query WorkspaceSQLite v3.45 (WASM Mode)
    QUICK INSERT:
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