WHERE Clause and Comparison Operators in SQL
The WHERE clause filters the rows produced by FROM, keeping only those for which a condition evaluates to true. Its job is deciding which rows get processed at all.
WHERE is how you tell SQL which rows to keep.
Every query using FROM starts with the full table — every row, nothing filtered yet. WHERE looks at each row one at a time, applies a test, and keeps only the rows where the test passes. Rows that fail get dropped before SELECT ever sees them.
As an analyst, this is how you narrow a broad dataset to just what you need. Finance wants orders above a dollar threshold. Marketing needs customers from one country. Operations is tracking a specific status. In every case, you write one condition after WHERE and SQL does the filtering:
SELECT name, email
FROM customers
WHERE country = 'US'SQL reads every row in customers. If country equals 'US', the row passes. If not, it's gone. Only matching rows reach the SELECT.
The comparison operators you'll use most: = (equals), <> (not equals), >, <, >=, <=. Anything that compares a column value to a number, a text string, or another expression uses one of these.
You can filter on any column in the table, even one you're not returning in SELECT. Filtering by price while only selecting name works fine.
The one thing that trips people up: text values need single quotes, numbers don't.
WHERE country = 'US' is correct. WHERE country = US gives you an error — SQL reads US as a column name. Numbers work without quotes: WHERE price > 100 is fine as written. Whenever the value you're filtering on is a word or phrase, wrap it in single quotes.
You want rows where status equals 'pending'. Which WHERE condition is correct?
9 WHERE Clause and Comparison Operators practice problems
Write a query to return the name and email of every customer registered in the United States.
Write a query to return the name and price of every product priced above $100.
Write a query to return the name and location of every department based in New York.
Write a query to return the name, original price, and discounted price for each qualifying product.
Write a query to return the ID and order total of every order currently in pending status.
Write a query to return the employee ID and salary amount for every record at or above $80,000.
Write a query to return the name and email of every user on the enterprise plan.
Write a query to return the name and total inventory value for every flagged product.
Write a query to return the ID and current status of every order that has not yet reached delivered status.
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