Question

I’m creating a new SQL Server 2008 database and I was always wondering, efficiency-wise, does it matter where I place the index column?

For instance, this:

--ID is primary key
CREATE TABLE tbl (ID INT, dtIn DATETIME2, dtOut DATETIME2, Type INT)
INSERT tbl VALUES
(1, '01:30', '02:00', 1),
(2, '02:30', '03:00', 1),
(3, '10:30', '11:00', 2)

CREATE INDEX idx_Type ON tbl(Type)

versus this:

--ID is primary key
CREATE TABLE tbl (ID INT, Type INT, dtIn DATETIME2, dtOut DATETIME2)
INSERT tbl VALUES
(1, 1, '01:30', '02:00'),
(2, 1, '02:30', '03:00'),
(3, 2, '10:30', '11:00')

CREATE INDEX idx_Type ON tbl(Type)

Answer 1

Well , it sounds interesting, depends on how you implement

Single column index -it does not matter 

Multi-column index - order of the column does matter in the index,
                     but not in the table

Ben has a proof here