SQL queries express the data you want from relational tables. Begin with explicit columns and filters, then add joins and grouping while checking row counts.
Before you begin
Use a disposable practice database with fictional tables. Record the database dialect and schema; syntax and features can differ between systems.
Work through the idea
Write down what one row represents and which keys should be unique. Predict a small result by hand before running a query. Compare row counts and totals after filtering or joining. Keep null handling, duplicate keys and date boundaries explicit so that a plausible output does not hide an incorrect query.
- Select the required fields
- Filter with a defined condition
- Check joins for duplicated rows
Practice task
Query fictional orders for one month and calculate totals by customer.
Check your result
Reconcile one customer total manually and explain handling of cancelled orders.
A mistake to avoid
Do not assume a join preserves the number of rows in the original table.
Keep a review note
Save the inputs or scenario, your reasoning and the result you checked. Explain one assumption and one case that needs more work. Revise the exercise after feedback and distinguish what you demonstrated from what remains untested.
Official reference for further study
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