SQL中用于处理条件逻辑和空值的函数

SQL Conditional and Null Handling Functions

In SQL, several functions and statements are essential for dealing with conditional logic and null values, including the CASE WHEN statement, COALESCE function, IF function (specific to some databases like MySQL), IFNULL function (also in MySQL), and the NULLIF function. Each of these tools is described below with usage examples.

CASE WHEN Statement

The CASE WHEN statement is a conditional expression in SQL that allows returning different values based on specified conditions. Its syntax is as follows:

CASE
  WHEN condition1 THEN result1
  WHEN condition2 THEN result2
  ...
  ELSE result
END

For example, using an employees table, we can categorize employees into 'High', 'Medium', or 'Low' salary brackets based on their salary values.

COALESCE Function

The COALESCE function takes multiple arguments and returns the first non-null value among them. If all arguments are NULL, it returns NULL. For instance, using the employees table, it can return an employee's phone number, email, or 'N/A' if both are NULL.

IF Function (MySQL)

The IF function in MySQL performs simple conditional checks and returns one of two results. Using the employees table, it can label employees as 'Above Average' or 'Average or Below' based on whether their salary is higher than 50,000.

IFNULL Function (MySQL)

The IFNULL function takes two arguments and returns the first argument's value if it's not NULL; otherwise, it returns the second argument's value. For example, selecting employees' names and phone numbers, it displays 'N/A' if the phone number is NULL.

NULLIF Function

The NULLIF function compares two expressions and returns NULL if they are equal; otherwise, it returns the value of the first expression. It's useful, for example, when calculating bonuses where you don't want to give a bonus to employees with a salary of 0.

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