How do I prepare for a SQL interview?

Work through realistic questions, read why each answer is right, then redo the ones you missed. This test has ten, taken from what interviewers commonly ask a beginner or a junior. Then read the postings: the 4,865 open jobs that name SQL show what employers expect on top of it.

Is this SQL test free?

Yes. It needs no account and no email, runs in your browser and sends nothing anywhere.

What does my score mean?

It is a self-check. Eight or more right means the basics are fresh; five to seven means a good base with gaps; under five means more practice first. It is not an official certificate.

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SQL test: 10 questions to check yourself

In plain words: SQL is the language used to ask a database questions, such as “which customers bought more than once this month”. Answer ten questions, press the button, and see which you got right and why.

4,865 open jobs name SQL today, with a median advertised pay of $186K. The questions are the kind that come up in a first interview.

1Which clause keeps only the rows that meet a condition before any grouping happens?HAVINGWHEREORDER BYLIMIT

2What does an INNER JOIN between customers and orders return?Every customer, with empty order columns when there is no orderOnly customers that have at least one matching orderEvery order and every customer, matched or notOnly the customers without orders

3You need the number of orders for each customer. Which query does it?SELECT customer_id, COUNT(*) FROM orders GROUP BY customer_idSELECT COUNT(*) FROM orders WHERE customer_idSELECT customer_id FROM orders COUNTSELECT customer_id, SUM(*) FROM orders

4What is the result of comparing a value with NULL using = (for example, price = NULL)?True when the price is emptyFalse for every rowUnknown, so the row is never returnedAn error

5Which keyword removes duplicate rows from the result?UNIQUEDISTINCTSINGLENODUP

6What is a primary key?A column that is always a numberA column, or set of columns, whose value identifies one row and is never repeated or emptyThe first column of a tableA password for the table

7What is the difference between UNION and UNION ALL?UNION is fasterUNION removes duplicate rows, UNION ALL keeps themUNION works on two tables onlyThere is no difference

8Which query returns the three highest salaries in a table named staff (in most databases)?SELECT salary FROM staff ORDER BY salary DESC LIMIT 3SELECT TOP salary FROM staff WHERE 3SELECT salary FROM staff GROUP BY 3SELECT 3 salary FROM staff

9What is an index for?It makes a table smallerIt lets the database find rows without reading the whole table, at the cost of slower writesIt checks that data is correctIt saves a copy of the table

10In a transaction, what does ROLLBACK do?Saves all changes made since the transaction beganUndoes all changes made since the transaction beganDeletes the tableLocks the database for everyone

Questions about this test

How do I prepare for a SQL interview?: Work through realistic questions, read why each answer is right, then redo the ones you missed. This test has ten, taken from what interviewers commonly ask a beginner or a junior. Then read the postings: the 4,865 open jobs that name SQL show what employers expect on top of it.
Is this SQL test free?: Yes. It needs no account and no email, runs in your browser and sends nothing anywhere.
What does my score mean?: It is a self-check. Eight or more right means the basics are fresh; five to seven means a good base with gaps; under five means more practice first. It is not an official certificate.

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