String to Integer Conversion in SQL

Introduction

SQL databases often require data to be in specific types. A common scenario is needing to convert string data (text) into integer (numeric) data for calculations, comparisons, or storage in integer-based columns. This tutorial explains how to perform string to integer conversions in SQL, covering common methods and considerations for error handling and data integrity.

Basic Conversion with CAST and CONVERT

The primary methods for type conversion in SQL are CAST and CONVERT. Both functions achieve the same goal: transforming a value from one data type to another.

CAST: This is the SQL standard function for type conversion. It takes the value to be converted as the first argument and the target data type as the second.
CONVERT: This function is specific to SQL Server and provides additional formatting options, particularly for dates and times. However, for simple integer conversions, it works similarly to CAST.

Here’s how to use them:

SELECT CAST('123' AS INT);

SELECT CONVERT(INT, '123');

In both examples, the string ‘123’ is converted to the integer 123. The result will be a numeric value that can be used in calculations.

Handling Conversion Errors

When converting strings to integers, it’s crucial to consider what happens when the string does not represent a valid integer. For example, trying to convert ‘abc’ or ‘12.34’ will result in an error.

SQL Server 2012 and later versions offer functions to handle these errors gracefully: TRY_CAST and TRY_CONVERT. These functions return NULL if the conversion fails instead of raising an error.

SELECT TRY_CAST('abc' AS INT); -- Returns NULL
SELECT TRY_CONVERT(INT, '12.34'); -- Returns NULL
SELECT TRY_CAST('456' AS INT); -- Returns 456

Using TRY_CAST or TRY_CONVERT prevents your queries from failing due to invalid input and allows you to handle these cases programmatically.

Converting Numeric Strings with Decimal Points

If the string contains a decimal point (e.g., ‘56.72’), a direct conversion to INT will fail. You need to first convert the string to a numeric data type that supports decimal places (like DECIMAL, NUMERIC, or FLOAT) and then cast or convert it to INT. This will truncate any decimal portion.

SELECT CAST(CAST('56.72' AS NUMERIC(19,4)) AS INT);

In this example:

CAST('56.72' AS NUMERIC(19,4)) converts the string ‘56.72’ to a numeric value with a precision of 19 digits and a scale of 4 decimal places.
CAST(... AS INT) then converts this numeric value to an integer, effectively truncating the decimal portion, resulting in 56.

When inserting into a table, you might only need the first conversion:

INSERT INTO TableB (MyIntCol)
SELECT CAST('56.72' AS NUMERIC(19,4)) FROM TableA;

This will implicitly convert the numeric value to an integer during insertion.

Database Specific Syntax

While CAST and CONVERT are widely supported, some database systems might offer alternative functions. For example, in Oracle, you might use TO_NUMBER:

SELECT TO_NUMBER('123') FROM dual;

The dual table is a dummy table in Oracle used for evaluating expressions. Always consult your specific database system’s documentation for the most appropriate method.

Best Practices

Error Handling: Always use TRY_CAST or TRY_CONVERT whenever possible to handle potential conversion errors gracefully and prevent query failures.
Data Validation: Before attempting a conversion, consider validating the string to ensure it contains only numeric characters. This can be done using regular expressions or string manipulation functions.
Precision and Scale: When converting numeric strings with decimal points, carefully consider the precision and scale of the intermediate numeric data type to avoid unexpected rounding or truncation.
Database Specific Syntax: Be aware of any database-specific functions or syntax that might be available for string to integer conversion.