--if/else语句
if 条件 then
SQL语句
else
SQL语句
elseif
SQL语句
end if;

create procedure test1(
  number int
)
  begin
  if number >  10 then
    select user();
  else
    select ‘please input a number > 10‘;
  end if;
  end$

mysql> create procedure test1(
    ->   number int
    -> )
    ->   begin
    ->   if number >  10 then
    ->     select user();
    ->   else
    ->     select ‘please input a number > 10‘;
    ->   end if;
    ->   end$
Query OK, 0 rows affected (0.00 sec)

mysql> call test1()$
ERROR 1318 (42000): Incorrect number of arguments for PROCEDURE test.test1; expected 1, got 0
mysql> call test1(11)$
+----------------+
| user()         |
+----------------+
| root@localhost |
+----------------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.01 sec)

mysql> call test1(1)$
+----------------------------+
| please input a number > 10 |
+----------------------------+
| please input a number > 10 |
+----------------------------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.01 sec)

mysql>