📝7 SQL Mistakes Every Beginner Makes (And How to Fix Them)

Learn the 7 common SQL mistakes beginners make and how to fix them for cleaner, more efficient queries.
7 SQL Mistakes Every Beginner Makes (And How to Fix Them)
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7 SQL Mistakes Every Beginner Makes (And How to Fix Them)

I still remember the first time my SQL query returned zero results. I'd checked the table, verified the column names, and re-read the query three times. Everything looked perfect. But the database disagreed—and I had no idea why.
Turns out, I'd written WHERE manager_id = NULL instead of WHERE manager_id IS NULL. One tiny word cost me an hour of debugging.
If you're learning SQL, you've probably hit similar frustrations. The good news? You're making the exact same mistakes every beginner makes. The better news? They're easy to fix once you know what to look for.
In this guide, I'll walk you through the 7 most common SQL mistakes that trip up beginners, why they happen, and exactly how to fix them. By the end, you'll avoid hours of head-scratching debugging and write cleaner, more reliable queries from day one.

Mistake #1: Using = NULL Instead of IS NULL

The Problem
This is the number one mistake beginners make, and it's deceptively simple. When you want to find rows with missing or undefined values, your first instinct is probably to write:
This query will run without errors—but it will always return zero results, even if there are NULL values in the table. Why? Because in SQL, NULL represents "unknown," and you can't compare unknown values using equals.
The Fix
Always use IS NULL to check for NULL values:
And use IS NOT NULL to find rows with values:
Pro Tip: Remember that NULL = NULL returns NULL (not TRUE), which is treated as FALSE in WHERE clauses. NULL isn't equal to anything—not even itself.

Mistake #2: Forgetting Quotes Around Strings

The Problem
Another common stumbling block: forgetting to wrap text values in quotes. You might write:
This throws an error because SQL thinks Electronics is a column name, not a text value. The database looks for a column called "Electronics" that doesn't exist.
The Fix
Always enclose string literals in single quotes:
Numbers don't need quotes:
Common Pitfall: Don't use double quotes for strings—in Standard SQL, double quotes are for identifiers (table/column names), not values.

Mistake #3: Mixing Up AND/OR Logic Without Parentheses

The Problem
When combining multiple conditions, beginners often write queries like this:
You might expect this to return all Electronics and Computers under $500. But because AND has higher precedence than OR, SQL reads it as:
"Get all Electronics (any price) OR (Computers under $500)"
This returns expensive Electronics you didn't want.
The Fix
Always use parentheses to make your intent crystal clear:
Now both categories must be under $500, which is what you wanted.
Rule of Thumb: Whenever you mix AND and OR, use parentheses—even if you understand precedence. It prevents errors and makes queries readable.

Mistake #4: Using SELECT * in Production Code

The Problem
When exploring data, typing SELECT * is convenient—it shows you everything:
But in production code, this is a performance killer. You're retrieving columns you don't need, wasting memory and network bandwidth. Worse, if someone adds columns to the table later, your application suddenly receives extra data it can't handle.
The Fix
Explicitly list only the columns you need:
Benefits:
  • Faster query execution
  • Lower memory usage
  • Code is self-documenting (shows exactly what you're using)
  • Future-proof against table schema changes
When to Use SELECT *: Only for quick data exploration or testing—never in application code.

Mistake #5: Using LIMIT Without ORDER BY

The Problem
You want to see a few sample records, so you write:
This seems fine, but without ORDER BY, you'll get random rows. Run it twice, and you might see different results. This makes debugging impossible and produces inconsistent application behavior.
The Fix
Always add ORDER BY when using LIMIT:
Now you consistently get the 10 most recent orders.
Use Cases:
  • Top N queries: "Get 5 highest-paid employees"
  • Pagination: "Show page 2 of products"
  • Recent items: "Display latest 20 posts"

Mistake #6: Ignoring NULL in Calculations

The Problem
You calculate a discount price like this:
It works fine—until you hit a product with a NULL discount. The entire calculation returns NULL because any arithmetic operation with NULL produces NULL.
So 100 - NULL = NULL, not 100.
The Fix
Use COALESCE() to provide a default value for NULL:
COALESCE(discount, 0) returns the discount if it exists, or 0 if it's NULL.
Other Common Cases:
  • total_sales / COALESCE(order_count, 1) (avoid division by zero)
  • COALESCE(middle_name, '') (convert NULL to empty string)

Mistake #7: Trying to Use Column Aliases in WHERE

The Problem
You create a calculated column and try to filter by it:
Error! SQL says price_with_tax doesn't exist. This confuses beginners because the alias is right there in the SELECT.
The Fix
SQL evaluates clauses in a specific order: WHERE runs before SELECT, so aliases don't exist yet when filtering happens.
Repeat the calculation in WHERE:
Good News: You CAN use aliases in ORDER BY because it runs after SELECT:

Conclusion: Write Better SQL from Day One

These seven mistakes account for probably 80% of the debugging time beginners spend on SQL. The good news? Now that you know them, you can avoid hours of frustration.
Quick Recap:
  • Use IS NULL, never = NULL
  • Wrap strings in single quotes
  • Use parentheses with AND/OR logic
  • Avoid SELECT * in production
  • Always pair LIMIT with ORDER BY
  • Handle NULL values in calculations
  • Don't reference aliases in WHERE
The secret to mastering SQL isn't memorizing syntax—it's understanding these common pitfalls and how to sidestep them. Keep this guide handy as you practice, and you'll write cleaner, more reliable queries from day one.
Next Steps: Try writing 10 practice queries focusing on each mistake. The more you practice, the more these patterns become second nature.

Practice Exercise

Try fixing this query—it contains 3 mistakes from this article:
Answers:
  1. Missing quotes: 'Sales'
  1. Missing parentheses: (department = 'Sales' OR salary > 50000)
  1. Wrong NULL check: IS NULL
  1. Bonus: Add ORDER BY to LIMIT
Did you catch them all?
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Oct 22, 2025