Comparing IF, IIF, and CASE

In this post, we'll compare and contrast the usage of IF, IIF, and CASE in MSSQL, which are essential for writing dynamic and conditional SQL statements.

IF vs IIF

The IF statement and the IIF function serve different purposes and have slightly different syntaxes.

	IF Statement	IIF Function
Use Case	Used for conditional branching in T-SQL, allowing you to execute different code blocks depending on whether a condition is true or false.	Returns one of two values, depending on whether a condition is true or false. This function essentially performs the same function as an IF...ELSE statement, but in a more concise syntax.
Syntax	IF condition BEGIN... END	IIF ( boolean_expression, true_value, false_value )
Null handling	The IF statement handles NULL according to the conditions specified within the statement. If a condition compares with NULL, it won't yield a TRUE or FALSE but a NULL.	If the boolean_expression is NULL, the IIF function returns NULL.
Scope	Can be used in batch or stored procedures to dictate program flow.	Primarily used in a SELECT statement for row by row processing. However, it can also be used in a WHERE or ORDER BY clause.
Functionality	Executes a set of statements based on the evaluation of a condition.	Evaluates a boolean expression and returns one of two values based on whether the expression resolves to true or false.

CASE vs IIF

The CASE expression and IIF function also serve different purposes.

	CASE Expression	IIF Function
Use Case	Returns a result based on the evaluation of a set of conditions. It's like a series of IF statements in a single SQL command.	Returns one of two values, depending on whether a condition is true or false.
Syntax	CASE WHEN condition THEN result [WHEN...ELSE...] END	IIF ( boolean_expression, true_value, false_value )
Null handling	Similar to IF, CASE handles NULL according to the conditions specified in the CASE statement. A comparison with NULL is neither true nor false, but NULL.	If the boolean_expression contains NULL, IIF returns NULL.
Scope	Can be used in any statement or clause that allows a valid expression, such as SELECT, UPDATE, DELETE, or SET statement.	Primarily used in a SELECT statement for row by row processing. However, it can also be used in a WHERE or ORDER BY clause.
Functionality	Evaluates a list of conditions and returns one of multiple possible result expressions.	Evaluates a boolean expression and returns one of two values based on whether the expression resolves to true or false.
Flexibility	Can evaluate multiple conditions, making it more flexible for complex conditional logic.	Can only evaluate one condition, limiting its use to simpler conditional logic.

It's important to remember that while IIF is functionally similar to using a CASE expression, IIF can only evaluate a single condition, while CASE can handle multiple

Understanding Statements, Expressions, and Functions

In addition to the comparison between IF, IIF, and CASE discussed above, it's also important to understand the differences between statements, expressions, and functions in MSSQL.

Statements

A SQL statement is a unit of execution in SQL. It performs a particular task or action in the database. Statements generally perform actions like creating or modifying database objects, inserting, updating, modifying or querying data, or controlling transactions and program flow.

For example, SELECT, INSERT, UPDATE, DELETE, and CREATE are all SQL statements. The IF in MSSQL is a control flow statement used for conditional branching.

Expressions

An expression is a combination of symbols—like constants, variables, operators, and function calls—that the programming language interprets and computes to produce another value. Essentially, an expression is a piece of code that produces a value when it's evaluated.

In SQL, an expression can be as simple as a single number or string. It can also be more complex, like a mathematical operation that involves multiple columns of a table, or a function call that manipulates and transforms data.

The CASE in SQL is an expression because it produces a value based on the evaluation of conditions. Similarly, the IIF function is also an expression.

Functions

Functions in SQL are a subtype of expressions because they also produce a value when called. However, functions encapsulate more complex operations that can be invoked using a specific syntax. Functions usually take in parameters, perform certain operations, and then return a result.

MSSQL provides many built-in functions to perform operations like string manipulation, date and time calculation, mathematical computations, and more. Functions can be used wherever expressions are allowed.

The IIF is a built-in function in MSSQL. It takes three arguments, evaluates the first one, and based on the result (true or false), it returns either the second or third argument.

In summary, while statements, expressions, and functions can often be used to perform similar tasks in SQL, they are used in different ways and have different roles in the SQL language. Understanding these differences can help you write more flexible, dynamic, and efficient SQL code.