SQL Server : Reuse calculated variable in select clause - sql

Question

I have the following table structure:
col1 col2 col3 col4
-------------------------------
aK Mbcd ABc defgh
col2, col3 and col4 columns are of type varchar(100) and col1 has type varchar(500).
I need a select query to have the output as following
col1 col2 col3 col4
-------------------------------
aK,Mb cd,A Bc,d efgh
Logic is explained as mentioned below:
In the result, Col2, col3 and col4 can have maximum 4 characters but col1 can have more than 4 characters upto 100.
If any column has more characters, last 4 characters will be retained in the same column and other extra columns will be concatenated with previous column's value separated by comma , and the same rule will be applied on the concatenated values as well.
I've written the following T-SQL statement. It works fine for last two columns. But I want to use new calculated value of col3 to strip out extra characters after adding some from col4
SELECT
CASE
WHEN X.Col4Length > 4
THEN concat(X.col3, ',', substring(x.col4, 0, X.Col4Length - 3))
ELSE X.col3
END AS col3,
CASE
WHEN X.Col4Length > 4
THEN substring(x.col4, X.Col4Length - 3, x.Col4Length)
ELSE X.col4
END AS col4
FROM
(SELECT
Col1, Col2, Col3, Col4,
Len(Col1) AS Col1Length,
Len(Col2) AS Col2Length,
Len(Col3) AS Col3Length,
Len(Col4) AS Col4Length
FROM
mytable) X

Answer 1

My try with a simple sub-query
with t1 as (
select 'aK' col1, 'Mbcd' col2, 'ABc' col3, 'defgh' col4
---
SELECT LEFT(col, LEN(col) - 12) col1,
RIGHT(LEFT(col, LEN(col) - 8), 4) col2,
RIGHT(LEFT(col, LEN(col) - 4), 4) col3,
RIGHT(col, 4) AS col4
FROM
(
SELECT col1+','+col2+','+col3+','+col4 AS col
FROM t1
) t;

Answer 2

You want to reuse calculated variables
There are two set-based /inline / adhoc approaches (and many more ugly procedural):
CTEs to do this for the whole set in advance
CROSS APPLY for the same on row level
Try it like this (CTE approach)
DECLARE #tbl TABLE(col1 VARCHAR(100),col2 VARCHAR(100),col3 VARCHAR(100),col4 VARCHAR(100));
INSERT INTO #tbl VALUES
('aK','Mbcd','ABc','defgh')
,('123456','abc','3456','123456789');
WITH ResolveCol4 AS
(
SELECT *
,RIGHT(col4,4) AS Col4_resolved
,col3 + ',' + CASE WHEN LEN(col4)>4 THEN SUBSTRING(col4,1,LEN(col4)-4) ELSE '' END AS col3_New
FROM #tbl
)
,ResolveCol3 AS
(
SELECT *
,RIGHT(col3_New,4) AS Col3_resolved
,col2 + ',' + CASE WHEN LEN(col3_New)>4 THEN SUBSTRING(col3_New,1,LEN(col3_New)-4) ELSE '' END AS col2_New
FROM ResolveCol4
)
,ResolveCol2 AS
(
SELECT *
,RIGHT(col2_New,4) AS Col2_resolved
,col1 + ',' + CASE WHEN LEN(col2_New)>4 THEN SUBSTRING(col2_New,1,LEN(col2_New)-4) ELSE '' END AS col1_New
FROM ResolveCol3
)
SELECT col1_new,Col2_resolved,Col3_resolved,Col4_resolved
FROM ResolveCol2
The result
aK,Mb cd,A Bc,d efgh
123456,abc,34 56,1 2345 6789