Ok - try these:
SELECT
TTT_Factureren = 0,
TTT_FactuurStatus = 0,
TTT_FactuurNummer = 0
FROM tblInvoicesProject
WHERE TTT_ProjectID = 26
AND TTT_FactuurStatus = 1
AND TTT_ID NOT IN (
SELECT SUBSTRING(CAST(X.TextDescription AS VARCHAR(100)),13,10)
FROM [500].dbo.frstx X
WHERE (LEFT(CAST(X.TextDescription AS VARCHAR(100)), 11) = 'Int.nr.TXT:')
)
SELECT
TTT_Factureren = 0,
TTT_FactuurStatus = 0,
TTT_FactuurNummer = 0
FROM tblInvoicesProject
WHERE TTT_ProjectID = 26
AND TTT_FactuurStatus = 1
AND TTT_ID NOT IN (
SELECT b.TTT_ID
FROM [500].dbo.frstx X
CROSS APPLY (SELECT ChosenRow = CASE WHEN X.TextDescription LIKE 'Int.nr.TXT:%' THEN 1 ELSE 0 END) a
CROSS APPLY (
SELECT TTT_ID = CASE WHEN ChosenRow = 1 THEN CAST(SUBSTRING(CAST(X.TextDescription AS VARCHAR(100)),13,10) AS INT) END) b
WHERE ChosenRow = 1
)
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden