Use the SQL Server BIT data type to define columns, variables, and parameters value of 1, 0, or NULL. 

Given their Yes/No nature, designer us the BIT type with flag and indicator columns: 

Converting BIT Values 

The string values TRUE and FALSE convert to BIT:  TRUE is converted to 1 and FALSE to 0. 

Converting any non-zero value promotes BIT to 1. 

Examples using SQL BIT 

Here is SQL to query all employees receiving a salary: 

SELECT NationalIDNumber , JobTitle , SalariedFlag FROM HumanResources.Employee WHERE SalariedFlag = 1;
SELECT NationalIDNumber 
 , JobTitle 
 , SalariedFlag 
FROM HumanResources.Employee 
WHERE SalariedFlag = 1;

Additional Resources 

To learn more about SQL data types, check out these useful resources: 

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