DEFAULT values in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the DEFAULT value in a SQL table column?

The DEFAULT value sets a preset value for a column when no value is provided during data insertion. It helps avoid NULLs and ensures consistent data.

beginner

How do you define a DEFAULT value for a column in SQL?

You use the DEFAULT keyword followed by the value when creating or altering a table column, for example: age INT DEFAULT 18.

intermediate

Can a DEFAULT value be a function in SQL? Give an example.

Yes, a DEFAULT value can be a function. For example, created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP sets the current time automatically.

beginner

What happens if you insert a row without specifying a column that has a DEFAULT value?

The column automatically gets the DEFAULT value specified in the table definition instead of NULL or an error.

intermediate

Is it possible to change the DEFAULT value of an existing column? How?

Yes, you can change it using the ALTER TABLE statement with ALTER COLUMN and SET DEFAULT, for example: ALTER TABLE users ALTER COLUMN age SET DEFAULT 21;.

What does the DEFAULT keyword do in SQL?

A Sets a preset value for a column when no value is provided

B Deletes a column from a table

C Changes the data type of a column

D Creates a new table

DEFAULT sets a preset value for a column if no value is given during insertion.

Which of the following is a valid DEFAULT value for a TIMESTAMP column?

A DEFAULT 0

B DEFAULT TRUE

C DEFAULT CURRENT_TIMESTAMP

D DEFAULT 'hello'

CURRENT_TIMESTAMP is a function that returns the current date and time, valid as a DEFAULT for TIMESTAMP.

If a column has DEFAULT 100 and you insert a row without specifying that column, what value will it have?

A 100

B 0

C NULL

D Error

The column will automatically get the DEFAULT value 100.

How do you change the DEFAULT value of an existing column in SQL?

A You cannot change DEFAULT values

B DROP COLUMN and add a new one

C UPDATE the table

D ALTER TABLE ... ALTER COLUMN ... SET DEFAULT ...

You use ALTER TABLE with ALTER COLUMN and SET DEFAULT to change the default value.

Can DEFAULT values be expressions or functions?

A No, only constants

B Yes, for example CURRENT_TIMESTAMP

C Only strings

D Only numbers

DEFAULT values can be functions like CURRENT_TIMESTAMP to set dynamic defaults.

Explain what DEFAULT values are in SQL and why they are useful.

Think about what happens if you forget to give a value for a column.

Describe how to set and change DEFAULT values for columns in SQL tables.

Consider both table creation and modifying existing tables.