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CAST

The CAST function is used to convert a value from one data type to another. It is commonly used when you need to change the data type of a value or a column to perform a specific operation, such as arithmetic or string concatenation.

Syntax​

CAST(expression AS data_type)
  • expression: The value or column you want to convert.
  • data_type: The target data type to which you want to convert the expression.

Examples​

Converting a value to a different data type​

SELECT CAST('TRUE' AS BOOLEAN) AS cast;

In this example, the CAST function is used to convert the string 'TRUE' to a boolean value.

Converting a column to a different data type​

Suppose you have a table called employees with the following structure:

CREATE TABLE employees (id INT, name TEXT, salary TEXT);

To calculate the total salary of all employees, you can use the CAST function to convert the salary column to a DECIMAL data type:

SELECT SUM(CAST(salary AS DECIMAL)) AS total_salary FROM employees;

Handling NULL values​

The CAST function can handle NULL values as well. If the expression is NULL, the result will be NULL:

SELECT CAST(NULL AS INTEGER) AS cast;

This query will return a NULL value.

Converting a value to a DATE or TIME data type​

The CAST function can also be used to convert strings to DATE or TIME data types:

SELECT CAST('2023-05-04' AS DATE) AS cast_date;
SELECT CAST('14:30:00' AS TIME) AS cast_time;

These queries will return a date and time value, respectively.

Limitations and Errors​

Some conversions may be impossible or result in an error. For example, trying to convert a non-numeric string to an integer will result in an error:

SELECT CAST('foo' AS INTEGER) AS cast;

This query will produce an error because the string 'foo' cannot be converted to an integer.