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I am confused concerning the quotation marks syntax ('single' vs "double") in QGIS.

I know something about the quotation marks and the way they are applied in Python scripting (i.e. comments, strings etc.).

There are even some related questions:

Nevertheless, I would like to clarify the details regarding the quotation marks ('single' vs "double") in the scope of QGIS.

For instance, if I type something in the Expression dialogue I will have a different output.

  • "something", it is written as

something1

The Output is NULL.

  • 'something', it is shown as

something2

The output is 'something' as a string.

What is the syntax or meaningful difference between quotation marks ('single' vs "double") in QGIS?

I do not know if there are any circumstances when other types of quotation marks are used in QGIS like the triple quotation marks in Python.

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4 Answers 4

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Double quotes indicate that the string represents the name of an attribute while a single quote is a literal string.

So in your first case you get NULL because you don't have an attribute called something.

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    My mnemonic for this is "Single for Strings, Double for Data". So to compare a data field with a string value you'd use "name" = 'Fred'
    – Spacedman
    Commented Nov 6, 2018 at 19:31
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Here is the answer directly from QGIS help:

column name "column name" → Value of the field column name, take care to not be confused with simple quote, see below

'string' → a string value, take care to not be confused with double quote, see above

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Double marks refer to columns in the attribute table, single marks to a string value. E.g. CASE WHEN "something" > 100 THEN 'a lot' ELSE 'not so much' END checks the size of integer values in your column and adds a string to fields in the attribute table based on that size.

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    Does this mean that the value of an attribute "something" is bigger than string '100'?
    – user30184
    Commented Nov 6, 2018 at 14:49
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    If the value of a field in column "something" is bigger than 100, then I write "a lot" into a new column, else the other text is written.
    – Erik
    Commented Nov 6, 2018 at 15:13
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    I asked because I do not know the syntax that QGIS is using. In databases '100' means a string and then '101'>'100' but also '99'>'100' because 9 comes after 1 in alphanumeric order. Does QGIS make difference and is it possible to compare by numbers WHEN "something" > 100?
    – user30184
    Commented Nov 6, 2018 at 17:10
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    Actually '100' really means a string, sorry, gonna edit it. Comparison of numbers is possible, yes.
    – Erik
    Commented Nov 7, 2018 at 8:05
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The explanation in the 3.34 docs made it very clear for me.

  • ' - single quote: A literal string. Functions usually take a literal string as input.
  • " - double quote: Gives you the value of the quoted element. So "height" gives you the value for the current row in the column "height"! Or returns NULL if no column "height" exists.
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