SQL Like function Broken? or Limited?

Question

I am trying to use the LIKE function to get data with similar names. Everything looks fine but the data I get in return is missing some values when I get back more than ~20 rows of data.

I have a very basic query. I just want data that starts with Lab, ideally for the whole day, or at least 12 hours. The code below misses some data and I cannot discern a pattern for what it picks to skip.

SELECT History.TagName, DateTime, Value  FROM History 
WHERE History.TagName like ('Lab%') 
AND Quality = 0
AND wwRetrievalMode = 'Full' 
AND DateTime >= '20150811 6:00'
AND DateTime <= '20150811 18:00'

To give you an idea of the data I am pulling, I have Lab.Raw.NTU, Lab.Raw.Alk, Lab.Sett.NTU, etc. Most of the data should have values at 6am/pm, 10am/pm, and 2am/pm. Some have more, few have less, not important. When I change the query to be more specific (i.e. only 1 hour window or LIKE "Lab.Raw.NTU") I get all of my data. Currently, this will spit out data for all tags and I get both 6am data and 6pm data, but certain values will be missing such as Lab.Raw.NTU at 6pm. There seem to be other data that is missing if I change the window for the previous day or the night shift, so I don't think it has to be with the data itself. Something weird is going on with the LIKE function but I have no idea what.
Is there another way to get the tagnames that I want besides like? Such as Tagname > Lab and Tagname <= Labz? (that gives me an error, so I am thinking not)

Please help.

Answer 1

It appears that you are using the Like operator correctly; that could be a red herring. Check the data type of the DateTime field. If it is character based such as varchar you are doing string comparisons instead of date comparisons, which could cause unexpected results. Try doing an explicit cast to ensure they are compared as dates:

DateTime >= convert(datetime, '20150811 6:00')