SQL Interview Question #10 | What is a Composite Key?

SQL Interview Question #10

What is a Composite Key in SQL?

A Composite Key is a combination of two or more columns that together uniquely identify a row in a table.

Individually, the columns may contain duplicate values.

But when combined, they create a unique record.

✅ Combination of multiple columns
✅ Uniquely identifies each row
✅ Used when one column alone is not sufficient

Example:

OrderDetails Table

## OrderID | ProductID

101 | 1
101 | 2
102 | 1
102 | 3

Here:

❌ OrderID is not unique

❌ ProductID is not unique

✅ OrderID + ProductID is unique

Therefore, (OrderID, ProductID) is a Composite Key.

💡 Interview Tip:

When one column cannot uniquely identify a row, combine multiple columns to create a Composite Key.

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