SQL Server – How to use a IF…ELSE statement in SQL

IF (SELECT product from product_table WHERE product_type = @productType) = 'HAM'
BEGIN 

SELECT product 
  FROM product_table
  LEFT OUTER JOIN meat_table b on b.product = product_table.product
 WHERE vendor = @vendor END 
ELSE

SELECT product
  FROM product_table
  LEFT OUTER JOIN veggie_table b on b.product = product_table.product
 WHERE vendor = @vendor