Possible to store value of one select column and use it for the next one?

Solution 1:

You need CROSS APPLY here, it can refer to outer references, no annoying subqueries or CTEs needed:

select col1, col2
from table1 as outer_table

-- can also have multi-row values
cross apply (values (complex_expression_1) ) as v1 (col1)
cross apply (values (expression_referring_to_col1) ) as v2 (col2)

-- alternate syntax, select without from returns a single row
cross apply (select complex_expression_1 as col1 ) AS v1
cross apply (select expression_referring_to_col1 as col2 ) as v2

-- you can also do anything you like in there, can be one or multiple rows
cross apply (
    select complex_expression_1 as col1 
    from othercomplexjoin as o
    where o.join_column = outer_table.join_column
) AS v1

Some more tricks you can do with APPLY:

1. Top 1 per group of child table:

A classic solution to the "top 1 per group" is to use row_number(). This can often result in huge scans, especially when the number of distinct outer values is small relative to the child table.

select
    o.id,
    lastPayment.Date
from order_header as o
join
( select *, row_number() over (partition by order_id order by date desc) as rn
 from payments
) as lastPayment on ...
where lastPayment.rn = 1

Instead we can do:

select
    o.id,
    lastPayment.Date
from order_header as o
cross apply
( select top (1) *
 from payments as p
 where p.order_id = o.id
 order by date desc
) as lastPayment

Note: OUTER APPLY conceptually replaces a left join, i.e. returns nulls instead of no rows.

2. Unpivoting

select
    o.id,
    customer.*
from order_header as o
cross apply ( values    -- This returns two rows for every order_header
    ( 'DeliveryCustomer', o.deliveryCustomer ),
    ( 'billingCustomer', o.billingCustomer )
) as customer (type, name)

3. Exploding out a row a variable number of times:

Say we want to take an amount, and split it into different rows. If the amount <= 50 then one row of amount, if > 50 then two rows, one of 50 and one of the rest:

select t.id, v.amount
from table as t
cross apply (
    select case when amount > 50 then 50 else amount end as amount
    union all
    select amount - 50   -- note this row will not appear if amount < 50
    where amount > 50
) v

Solution 2:

A sub-query is the simplest form. You can nest sub-queries to any level you like performing calculations along the way:

select 
    ComplexValue1 as FirstColumn
    , ComplexValue1 + ComplexValue2 as SecondColumn
from (
  select 
    {complex query returns a value} ComplexValue1
    , {complex query returns a value} ComplexValue2
    , {any other required columns}
  from SomeTable
) X;

Note: a CTE is essentially the same thing just prettier (maybe).

A lateral join is another option as it allows you to reference columns in the base table and perform calculations on them which are then available for use in the resultset:

select
    X.ComplexValue1 as FirstColumn
    , X.ComplexValue1 + ST.ComplexValue2 as SecondColumn
from SomeTable ST
cross apply (select {complex query returns a value referencing ST}) as X (ComplexValue1);