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How to query for a field being an empty string?

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I am trying to perform a read query that a field either matches a fixed value OR is empty. This is using the Python dds library, though I hope that doesn't affect the answer.

What I have tried: 

(identity = 'saluser@2e2a97a8cda0') OR (identity = '')

The problem appears to be in the second part

 identity = ''

 Using double quotes instead of single quotes doesn't help.

If I add a space between two single quotes then the query is built, but does not do what I want. Any suggestions?

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10 hours ago, rowen said:

Dear Rowen,

You can create query condition with the any string. Our python API  is working fine with  both string syntax (i.e single quoted and double quoted) .

I would like to explain create_querycondition API in detail, it may help you to understand your problem.

 create_querycondition(self, masks=DDSMaskUtil.all_samples(), expression = '', parameters=[])

Parameter Details:

    reader (DataReader) – Source DataReader
    masks (list) – list of masks (default: DDSMaskUtil.all_samples()
    expression (string) – Expression (query string, which must be a subset of the SQL query language.)
    parameters (list) – list of parameters in string (a sequence of strings which are the parameter values used in the SQL query string (i.e., the “%n” tokens in the   expression). The number of values in query_parameters must be equal or greater than the highest referenced %n token in the query_expression.) 

Example code snipped:

1. I have created a parameter list for saleuser1 and empty string.

param = ["saleuser1", ""]

2. Now i have created a query expression i.e. 'identity=%0 OR identity=%1' it takes paraments from the param list. i have passed query_expression and query_parameters into QueryCondition API.
    qc = QueryCondition(reader, DDSMaskUtil.all_samples(),   'identity=%0 OR identity=%1', param)


With best regards,



10 hours ago, rowen said:



10 hours ago, rowen said:




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Thank you for the suggestion. Unfortunately I tried it and was not able to get it work: yes the query was accepted, but it did not behave as it was supposed to.

Fortunately this is a short-term issue. Eventually I can get rid of the test for the empty string, at which point I'll be able to use a query again.

I am quite surprised that this is accepted:

qc = QueryCondition(reader, mask, "identity=%0 OR identity=%1", ["foo", ""])

but this is not:

qc = QueryCondition(reader, mask, "identity='foo' OR identity=''")

I realize for real SQL queries it is important to sanitize inputs, but is SQL injection a problem for DDS read queries? If so I'll start doing that, but otherwise I find it much clearer to specify the values directly in the query string.

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