how to check if exists in sql code example

Example 1: if exists sql server

IF EXISTS (SELECT * FROM tblGLUserAccess WHERE GLUserName ='xxxxxxxx') 
BEGIN
   SELECT 1 
END
ELSE
BEGIN
    SELECT 2
END

Example 2: t-sql check if data exists

if exists(select 1 from INFORMATION_SCHEMA.TABLES T 
              where T.TABLE_NAME = 'Bookings') 
begin
    drop table Bookings
end
GO

create table Bookings(
  FlightID    int identity(1, 1) primary key,
  TicketsMax    int not null,
  TicketsBooked int not null
)
GO

insert  Bookings(TicketsMax, TicketsBooked) select 1, 0
insert  Bookings(TicketsMax, TicketsBooked) select 2, 2
insert  Bookings(TicketsMax, TicketsBooked) select 3, 1
GO

select * from Bookings

Example 3: Check if value exisits update or insert sQL

IF EXISTS (SELECT * FROM Bookings WHERE FLightID = @Id)
BEGIN
    --UPDATE HERE
END
ELSE
BEGIN
   -- INSERT HERE
END