The begin and end keywords enclose a series of statements so they are treated as a unit by control-of-flow constructs like if...else. A series of statements enclosed by begin and end is called a statement block.
The syntax of begin...end is:
begin statement block end
For example:
if (select avg(price) from titles) < $15 begin update titles set price = price * 2 select title, price from titles where price > $28 end
Without begin and end, the if condition applies only to the first Transact-SQL statement. The second statement executes independently of the first.
begin...end blocks can nest within other begin...end blocks.