How to replace null field values with an empty string? - sql

Question

I need to replace the null values in the Estimate Number field with a blank string. I have tried the code below and the values still appear as null. Any thoughts? thanks
Pro Number Estimate Number
10271943 NULL
10271944 NULL
10271945 NULL
10271946 NULL
10271948 94606
SELECT a.AAAREFNUMVALUE AS "Pro Number",
(SELECT TOP 1 isnull(a2.AAAREFNUMVALUE,'')
FROM dbo.AAATOREFNUMS a2
WHERE a2.AAATRANSPORTTABLE = a.AAATRANSPORTTABLE AND
a2.AAAREFNUMTYPE = 4
ORDER BY a2.AAAREFNUMVALUE
) AS "Estimate Number"
FROM dbo.AAATOREFNUMS a
INNER JOIN dbo.AAATODATES d ON a.AAATRANSPORTTABLE = d.AAATRANSPORTTABLE
WHERE a.AAAREFNUMTYPE = 1 AND d.AAADATETYPE = 1
GROUP BY a.AAAREFNUMVALUE,a.AAATRANSPORTTABLE,a.AAAREFNUMTYPE;

Answer 1

The problem occurs when the subquery returns no values. The isnull() needs to go outside the subquery:
ISNULL( (SELECT TOP 1 a2.AAAREFNUMVALUE
FROM dbo.AAATOREFNUMS a2
WHERE a2.AAATRANSPORTTABLE = a.AAATRANSPORTTABLE AND
a2.AAAREFNUMTYPE = 4
ORDER BY a2.AAAREFNUMVALUE
), ''
) AS "Estimate Number"
Note that this is a situation where ISNULL() is preferred over COALESCE(), because the SQL Server implementation of COALESCE() evaluates the first argument twice when it is not NULL.
However, you might find the query easier to express and faster to run if you use window functions instead:
SELECT DISTINCT a.AAAREFNUMVALUE AS "Pro Number",
COALESCE(a.AAAREFNUMVALUE_4, '') as "Estimate Number"
FROM (SELECT a.*,
MAX(CASE WHEN a.AAAREFNUMTYPE = 4 THEN a.AAAREFNUMVALUE END) OVER (PARTITION BY a.AAATRANSPORTTABLE) as AAAREFNUMVALUE_4
FROM dbo.AAATOREFNUMS a
) a INNER JOIN
dbo.AAATODATES d
ON a.AAATRANSPORTTABLE = d.AAATRANSPORTTABLE
WHERE a.AAAREFNUMTYPE = 1 AND d.AAADATETYPE = 1 ;

Answer 2

Try case, just make the adjusts:
CASE
(SELECT TOP 1 a2.AAAREFNUMVALUE
FROM dbo.AAATOREFNUMS a2
WHERE a2.AAATRANSPORTTABLE = a.AAATRANSPORTTABLE AND
a2.AAAREFNUMTYPE = 4
ORDER BY a2.AAAREFNUMVALUE) as Estimate Number
WHEN NULL THEN ''
ELSE
a2.AAAREFNUMVALUE
END as "Estimate Number"

Answer 3

You may try this.
SELECT a.AAAREFNUMVALUE AS "Pro Number",
ISNULL((SELECT TOP 1 isnull(a2.AAAREFNUMVALUE,'')
FROM dbo.AAATOREFNUMS a2
WHERE a2.AAATRANSPORTTABLE = a.AAATRANSPORTTABLE AND
a2.AAAREFNUMTYPE = 4
ORDER BY a2.AAAREFNUMVALUE
),'') AS "Estimate Number"
FROM dbo.AAATOREFNUMS a
INNER JOIN dbo.AAATODATES d ON a.AAATRANSPORTTABLE = d.AAATRANSPORTTABLE
WHERE a.AAAREFNUMTYPE = 1 AND d.AAADATETYPE = 1
GROUP BY a.AAAREFNUMVALUE,a.AAATRANSPORTTABLE,a.AAAREFNUMTYPE;