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SQL Intermediate: Queries That Survive Real Data
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SQL Intermediate: Queries That Survive Real Data

Check Yourself: Schema Design That Holds

Five scenario questions on normal forms, price snapshots, composite UNIQUE, CHECK and ON DELETE. If you miss more than one, reread the lessons in this section.

5 min SQLDatabasesPerformance Optimization Intermediate
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1. A sales table stores the products of each sale in one text column, separated by commas. What is the main problem?
The column uses too much disk space. The database cannot check the names or link them to a product table, and counting them needs extra work. Text columns cannot be searched. SELECT cannot read a text column.
2. A discount_percent column has CHECK (discount_percent BETWEEN 1 AND 90). An INSERT with a NULL discount succeeds. Why?
PostgreSQL skips CHECK rules for the first row. BETWEEN treats NULL as zero. A CHECK passes when its result is NULL. Only false fails it. The CHECK was ignored because it has a name.
3. An order from August shows a higher total after the product price changes in October. What change to the design fixes that?
Store the price on the order line at the time of sale and read the total from it. Join the order lines to products with a LEFT JOIN. Add a CHECK that prices cannot change. Remove the price column from products.
4. A table has UNIQUE (code, region), and two rows are inserted with the same code and a NULL region. What happens?
Both rows are refused, because NULL is not allowed. Both rows are accepted, because two NULLs are not equal to each other. The first row is replaced by the second. The second row is refused.
5. Support tickets must stay in the database when a customer is deleted, with no customer attached. Which foreign key action fits?
ON DELETE RESTRICT No foreign key at all. ON DELETE SET NULL, on a customer_id column that allows NULL. ON DELETE CASCADE

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Five scenario questions on normal forms, price snapshots, composite UNIQUE, CHECK and ON DELETE. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
10/01/2026
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