Scheduled datetime conversion error (SQL) - sql-server

Question

When trying to convert a datetime field to get rid of NULL's I am getting the error that the datetime cannot be converted.
This query is populating another database where the column datatype is date.
This job runs perfectly fine when not scheduled, but results in an error during the scheduled insert job.
Any ideas?
, (SELECT
CASE
WHEN MIN([Discount Ledger].[Posting Date]) IS NOT NULL
THEN CONVERT(VARCHAR(50), MIN([Discount Ledger].[Posting Date]), 121)
ELSE '-'
END
FROM
[Wings$Discount Ledger Entry] AS "Discount Ledger"
LEFT JOIN
[Wings$Periodic Discount] AS "MixMatchHeader" ON [Discount Ledger].[Offer No_] = [MixMatchHeader].[No_]
LEFT JOIN
[Wings$Periodic Discount Line] AS "MixMatch" ON [MixMatchHeader].[No_] = [MixMatch].[Offer No_]
LEFT JOIN
[Wings$Item_Special Group Link] AS "Special Group" ON [MixMatch].[No_] = [Special Group].[Special Group Code]
LEFT JOIN
[Wings$Store Price Group] AS "Store Price Group" ON [MixMatch].[Price Group] = [Store Price Group].[Price Group Code]
WHERE
[MixMatch].[No_] = [Item Ledger].[Item No_]
AND [MixMatch].[Variant Code] = [Item Ledger].[Variant Code]
AND [Store Price Group].[Store] = [Item Ledger].[Location Code]) AS "Discount Start Date"

Answer 1

You can do this using ISNULL. Since you are looking for the MIN date, I will use GETDATE() as the argument/replacement value for NULL. Unless your data has future dates, then GETDATE() should function as the MAX value. Unless you want the CASE statement to evaluate the NULL values as the MIN. In that case, you can use an empty string '' or '1900-01-01' instead of GETDATE().
, (SELECT
CASE
WHEN
MIN([Discount Ledger].[Posting Date]) IS NOT NULL
THEN CONVERT(varchar(50),MIN(ISNULL([Discount Ledger].[Posting Date],GETDATE())),121)
ELSE '-'
END
FROM [Wings$Discount Ledger Entry] AS "Discount Ledger"
LEFT JOIN [Wings$Periodic Discount] AS "MixMatchHeader" ON [Discount Ledger].[Offer No_] = [MixMatchHeader].[No_]
LEFT JOIN [Wings$Periodic Discount Line] AS "MixMatch" ON [MixMatchHeader].[No_] = [MixMatch].[Offer No_]
LEFT JOIN [Wings$Item_Special Group Link] AS "Special Group" ON [MixMatch].[No_] = [Special Group].[Special Group Code]
LEFT JOIN [Wings$Store Price Group] AS "Store Price Group" ON [MixMatch].[Price Group] = [Store Price Group].[Price Group Code]
WHERE [MixMatch].[No_] = [Item Ledger].[Item No_] AND [MixMatch].[Variant Code] = [Item Ledger].[Variant Code] AND [Store Price Group].[Store] = [Item Ledger].[Location Code])
AS "Discount Start Date"