SQL for QA Testing | Database Testing + Interview Q&A

Learn how select and from build a basic query to retrieve data: select defines columns (use * for all) and from specifies the table, returning all rows.

Master the where clause to filter data by conditions, returning only rows that meet criteria such as score or country.

Explore the order by clause to sort results by a specified column using ascending or descending order, with examples on the score column in the customer table.

Learn how to use the group by clause to aggregate data by a column, such as total score by country, with aliases for clear output.

Learn to filter data after aggregation with having, group by, and where clauses to compute the average score by country, excluding zeros and returning countries with an average above 430.

Learn to use the distinct command in SQL to remove duplicate values from a result set by executing select distinct country from customer.

Learn how to define a database structure with ddl by creating a person table, listing id, person name, birth date, and phone, applying not null and primary key constraints.

Learn to use the alter command to modify a table, such as adding an email column to the person table with not null, then dropping a column to see effect.

Learn how the drop command removes a table entirely, deletes all data, and makes recovery impossible, then compare drop to create and alter within ddl.

Master the insert command in SQL: add single or multiple rows with optional column lists, ensure value counts match, and move data between tables using insert into select.

Learn how to use the SQL update command to modify existing rows with set and where clauses, and handle null values safely.

Learn how to safely delete rows with the SQL delete command, using a where clause to avoid removing all data, and compare it with truncate for faster data removal.

Learn how to combine tables in sql by columns or rows, using inner, left, right, full joins for horizontal data and union, union all, except, intersect for vertical stacking.

Learn how inner join returns only the matching rows from two tables, giving common data between customers and orders, with an example query selecting customer id, first name, and sales.

Learn full join in SQL to return all rows from both tables, including matching and non-matching data, using the on clause, and see nulls for non-matches.

Learn how a left join returns all rows from the left table and only matching rows from the right table, with nulls when there is no match.

Learn right join by returning all rows from the right table with matching rows from the left table, using right join syntax and mindful of table order and nulls.

Identify a primary key as the column or combination of columns that uniquely identifies each row, allows one per table, cannot contain null values, and relates tables via foreign keys.

Explore how a foreign key creates relationships between tables, enforces referential integrity, handles duplicates and null values, and distinguishes it from a primary key, with department and employee examples.

Understand how the unique key constraint ensures distinct values across rows while allowing nulls, such as the email column, and how it differs from the primary key.

Understand delete, truncate, and drop: delete removes rows (specific or all) with rollback; truncate clears all rows fast and cannot be rolled back; drop removes the table with no rollback.

Remove duplicates with union, but union all keeps duplicates and runs faster. Use union for unique data and union all for results, with same column count and compatible data types.

Identify duplicate emails in the employee table using group by, count, and having to filter groups with count greater than one for data integrity.

Learn how to display the first FIU records from a table in SQL Server using the top keyword, selecting all columns from the employees table.

Learn to find the highest salary by department using group by and max, with practical query examples for salary analysis and testing dashboards.