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Constraints

SQL DEFAULT

What is SQL DEFAULT?

A DEFAULT constraint in SQL is a database constraint that specifies a default value for a column when no value is explicitly provided during an INSERT operation. It ensures that, if a value is not provided for a column, the default value is automatically used. The DEFAULT constraint helps maintain data consistency and prevents NULL values in columns that should have a specific default value.

When you would use it

You would use a DEFAULT constraint in SQL when you need to:

  1. Ensure Data Consistency: Guarantee that a default value is automatically used when no explicit value is provided during an INSERT operation, preventing NULL values in specific columns.

  2. Streamline Data Entry: Simplify data entry by automatically assigning default values to columns, reducing the need for users to explicitly provide values for those columns.

  3. Maintain Data Integrity: Ensure that certain columns consistently have predefined default values, reducing the risk of data inconsistencies.

  4. Implement Business Rules: Enforce specific business rules that require the use of default values in particular situations.

Syntax

The syntax for defining a DEFAULT constraint in SQL is as follows:

CREATE TABLE table_name (
    column1 data_type DEFAULT default_value,
    column2 data_type,
    -- ...
);
  • table_name: The name of the table for which you're creating the DEFAULT constraint.
  • column1, column2, ...: The columns in the table.
  • data_type: The data type of the column.
  • default_value: The default value to be used when no explicit value is provided for the column.

Parameter values

  • table_name: The name of the table for which you're defining the DEFAULT constraint.
  • column1, column2, ...: The columns within the table to which you want to apply the DEFAULT constraint.
  • data_type: The data type of the column, specifying the type of data it can store.
  • default_value: The default value to be used when no value is explicitly provided during an INSERT operation. This can be a literal value, an expression, or a function call.

Example query

Here's an example SQL query that creates a "products" table with a DEFAULT constraint that sets the "discount" column to 0.0 if no discount value is provided during an INSERT operation:

CREATE TABLE products (
    product_id INT PRIMARY KEY,
    product_name VARCHAR(100),
    price DECIMAL(10, 2),
    discount DECIMAL(5, 2) DEFAULT 0.0,
    -- Other columns
);

Example table response

The "products" table is created with a DEFAULT constraint on the "discount" column, setting it to 0.0 by default. The table might contain data like this:

product_idproduct_namepricediscount
1Laptop899.990.10
2Smartphone499.990.00
3Tablet299.990.05

In this example, when no discount value is provided during an INSERT operation, the DEFAULT constraint automatically assigns a value of 0.0 to the "discount" column.

Use cases

  1. Ensure Data Consistency: To guarantee that a default value is automatically used when no value is explicitly provided during an INSERT operation, preventing NULL values in specific columns.

  2. Streamline Data Entry: To simplify data entry by automatically assigning default values to columns, reducing the need for users to explicitly provide values for those columns.

  3. Maintain Data Integrity: To ensure that certain columns consistently have predefined default values, reducing the risk of data inconsistencies.

  4. Implement Business Rules: To enforce specific business rules that require the use of default values in particular situations.

SQL Languages Availability

The concept of a DEFAULT constraint is available in most SQL-based relational database management systems (RDBMS), including but not limited to:

  • MySQL
  • PostgreSQL
  • Microsoft SQL Server
  • Oracle Database
  • IBM Db2
  • SQLite

While the core concept of DEFAULT constraints is universal, the specific implementation and syntax may vary between different database systems. Always consult your specific RDBMS documentation for precise details on using DEFAULT constraints.

Related

SQL CHECK

SQL Create Index

SQL Subqueries

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