Data Engineering Associate with Databricks Practice Exam

What is the correct syntax to add a constraint on a table in SQL?

1. ADD CONSTRAINT valid_date CHECK (date > '2020-01-01')

2. ALTER TABLE purchase_dates ADD CHECK (date > '2020-01-01')

3. ALTER TABLE ADD CONSTRAINT valid_date (date > '2020-01-01')

4. ALTER purchase_dates ADD CONSTRAINT valid_date CHECK (date > '2020-01-01')

The correct answer is: ADD CONSTRAINT valid_date CHECK (date > '2020-01-01')

The correct syntax to add a constraint on a table in SQL involves using the `ALTER TABLE` statement, which is important because it allows you to modify an existing table structure. The appropriate syntax includes specifying the constraint type, in this case, a `CHECK` constraint, and providing the condition that the data must meet. In option A, the syntax correctly states that a `CONSTRAINT` named `valid_date` is being added with the `CHECK` condition that stipulates the date must be greater than January 1, 2020. This defines a rule that the entries in the `date` column must comply with, effectively ensuring data integrity by preventing invalid data entry. Other options do not follow the correct structure. For instance, while options B and D start with `ALTER TABLE`, they have errors in the formatting and sequencing of their clauses or the table name usage. Option C fails entirely to define the table being altered, which is essential for the SQL command to execute successfully. Thus, option A efficiently captures the necessary components and adheres to the proper syntax for adding a constraint.