Question

Why does this statement succeed?

    SELECT CAST('08:50' as time)

But this one fails? tmrec is an nvarchar(6) column and contains the same value '08:50'. This is making me crazy since last 1 hour.

    SELECT   TOP 1 CAST(tmrec as time) 
    FROM     Instr 
    WHERE    igrp = 'JD' 
    ORDER BY ino , smallin

This screenshot shows the 1st query’s result. It contains 08:50. Yet the 2nd query throws error.

Edit:

Even this doesn’t work which guarantees that conversion is applied only on fetched records:

    SELECT CAST( tmrec as time)
    FROM
         (
            SELECT TOP 1 tmrec
            FROM   [ccwise-courts].[dbo].[INSTR]
            WHERE  igrp = 'JD' 
            ORDER BY ino , smallin
         ) v

Answer 1

Generally, to look for bad data, you can use a query like this:

SELECT TOP(100) '-->' + REPLACE(tmrec, ' ', '*') + '<--bad'
FROM Instr
WHERE ISDATE(tmrec) = 0

And if you still cannot make it out, you can list out the specific ASCII code of the characters involved (here I go up to 6 per the question):

SELECT TOP(100) '-->' + REPLACE(tmrec, ' ', '*') + '<--bad',
  Char1 = ascii(substring(tmrec,1,1)),
  Char2 = ascii(substring(tmrec,2,1)),
  Char3 = ascii(substring(tmrec,3,1)),
  Char4 = ascii(substring(tmrec,4,1)),
  Char5 = ascii(substring(tmrec,5,1)),
  Char6 = ascii(substring(tmrec,6,1))
FROM Instr
WHERE ISDATE(tmrec) = 0

There is this Connect item that deals with SQL Server processing CAST on the SELECT clause before the WHERE filter has been applied. To overcome that, as also noted in the bug report, you can use a CASE statement:

CAST(CASE WHEN ISDATE(tmrec)=1 THEN tmrec END as time)