Structured Query Language (SQL) is a programming language used to manage and manipulate relational databases. SQL Default is a feature of SQL that allows you to specify default values for columns in a table. When a new row is inserted into the table and a value is not specified for a column with a default value, the default value is used instead.
SQL Default is useful for ensuring that certain columns always have a value, even if it is not explicitly provided. It can also simplify the process of inserting new rows into a table by reducing the number of values that need to be specified.
To specify a default value for a column in a table, you can use the DEFAULT keyword followed by the value you want to use as the default. For example, the following SQL creates a table with a column named "status" that has a default value of "active":
CREATE TABLE users ( id INT PRIMARY KEY, name VARCHAR(50), status VARCHAR(10) DEFAULT 'active' );
When a new row is inserted into the "users" table and a value is not provided for the "status" column, the default value of "active" will be used.
You can also use SQL Default to specify default values for columns that are not explicitly included in an INSERT statement. For example, the following SQL inserts a new row into the "users" table without specifying a value for the "status" column:
INSERT INTO users (id, name) VALUES (1, 'John Doe');
Because a value was not provided for the "status" column, the default value of "active" will be used.
The default value you specify for a column must be compatible with the data type of the column. For example, if a column has a data type of INTEGER, the default value must be an integer. The following table shows some examples of default values for different data types:
| Data Type | Default Value |
|---|---|
| INTEGER | 0 |
| VARCHAR | '' (empty string) |
| DATE | CURRENT_DATE |
You can also use expressions as default values. For example, the following SQL creates a table with a column named "created_at" that has a default value of the current date and time:
CREATE TABLE posts ( id INT PRIMARY KEY, title VARCHAR(100), content TEXT, created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
When a new row is inserted into the "posts" table and a value is not provided for the "created_at" column, the current date and time will be used as the default value.
SQL Default is a useful feature of SQL that allows you to specify default values for columns in a table. It can simplify the process of inserting new rows into a table and ensure that certain columns always have a value. By understanding how to use SQL Default, you can make your SQL code more efficient and effective.