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I've a problem when using Select features using an expression. I've an attribute table with many features: It's a shapefile from a previous CSV table. The data are localities for water analysis, where several parameters appear. But I only want few of those locations.

What I do is:in Attribute table, I open Select by Expression, Then in Fields and Values, I select my column of interest "Parameters", and from Load Values- All Unique button, I choose the names I want to pick : alfa- hexaclorociclohexano, beta-hexaclorociclohexano, gamma-hexaclorociclohexano.

The expression I use is:

"Parametro"  =  'alfa-Hexaclorociclohexano'  +   'beta-Hexaclorociclohexano'  +  'delta-Hexaclorociclohexano'  +  'gamma-Hexaclorociclohexano'

But I can't select my values... If I just pick every value independently, i.e.: 'alfa-Hexaclorociclohexano', they are selected. Then, what my problem can be ?, should I not use the + symbol ? I also tried with the And symbol, but it's not working either. Should I try another option like the Query Builder ?

Select by Expression seemed the easier option to me but I'm not able to continue.

select by expression

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There are two possibilities when you want to select several value.

The first is to use the OR operator :

"Parametro" = 'alfa-Hexaclorociclohexano' OR "Parametro" = 'beta-Hexaclorociclohexano' OR "Parametro" = 'delta-Hexaclorociclohexano' OR "Parametro" = 'gamma-Hexaclorociclohexano'

You need to repeat the field name each time (this also work if you want to use several field)

The second is to use the IN operator :

"Parametro" IN ('alfa-Hexaclorociclohexano','beta-Hexaclorociclohexano','delta-Hexaclorociclohexano','gamma-Hexaclorociclohexano')

Work only for one field but easier to read and type (of course if you need two or more field you can join several with AND or OR...)

  • Thanks for all answers. Yes, it works. I select the pollutants I want from my "Parametro" field. But the selected elements don't get highlighted in my map, I don't know why. At the end, I'd like to save the highlighted elements, to work only with them – prigual Jun 14 '18 at 14:05
  • Don't know why it's not highlighted but if do a "save as" and check the "only selected" box you should get what you want – J.R Jun 14 '18 at 14:15
  • Hi again. I'd like to ask a problem I'm having doing exactly the same. Same situation, same data. I'm using again this expression but the result is 0. I used both the expression with OR and with IN, but it says Output preview: 0. what can it be this time ? I think I haven't changed anything @J.R – prigual Aug 22 '18 at 21:37
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If I understand your data correctly, you should use OR, so it would read:

"Parametro" = 'alfa-Hexaclorociclohexano' OR "Parametro" = 'beta-Hexaclorociclohexano' OR "Parametro" = 'delta-Hexaclorociclohexano' OR "Parametro" = 'gamma-Hexaclorociclohexano'

That way either alpha, beta, delta or gamma gets selected. If you use AND all 4 categories have to be true.

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