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I am not a new QGIS user, however, I am still confused concerning the quotation marks syntax ('single' vs "double").

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:

• Single quotes vs. double quotes in Python [closed]


• Is there any difference between “string” and 'string' in Python? [duplicate]

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

The Output is NULL.

• 
  'something', it is shown as

The output is 'something' as a string.

So, my questions are:

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


2. Are there any other circumstances when other types of quotation marks are used in QGIS, for instance, triple quotation marks as in Python?

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.

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

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' checks the size of integer values in your column and adds a string to fields in the attribute table based on that size.