IF...ELSE

Conditionally execute T-SQL statements.

Syntax
      IF boolean_expression
      { sql_statement | statement_block }
    [ ELSE
      { sql_statement | statement_block } ]

Key
   boolean_expression
        An expression that returns TRUE or FALSE.
        If Boolean_expression contains a SELECT statement,
        it must be enclosed in parentheses.

   sql_statement | statement_block
        Any T-SQL statement or statement grouping as
        defined with a statement block.
        Unless a statement block is used, the IF or ELSE condition
        can affect the performance of only one T-SQL statement.

        Use BEGIN and END to define a statement block.

Example

IF exists (select *
	        from sales
	        where customer_id = 123)
  Print 'Record exits'
ELSE
  Print 'Record doesn''t exist'

“IF you can keep your head when all about you, Are losing theirs and blaming it on you...” ~ Rudyard Kipling 1899

Related:

EXISTS
Equivalent Oracle command: Select into a cursor then: IF cursorname = 123 Then...