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In QGIS 3.6.3 I'm trying to create a Virtual Layer obtained by merging 12 shapefiles (with the columns named and ordered in the same way) using the expression

SELECT *
FROM _January, _February, _March, _April, _May, _June, _July, _August, _September,
_November, _October, _December

but the software crashes.

If I wanted to create a Virtual Layer by taking data from a single column called "TEST", how can I modify the expression? I could not find the correct form.

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    A little more additional information, such as type of software and what tools you are using for the merge, might help provide an answer. – Kingfisher Jun 14 '19 at 12:47
  • thanks @kazuhito, I tried the formula (2) and it works. I asked why when I tried it I couldn't get it to work (because I was wrong to write the command to join all the layers) thanks again for help – Fabrizio Jun 21 '19 at 11:23
  • Please don't add "thank you" as an answer. Once you have sufficient reputation, you will be able to vote up questions and answers that you found helpful. - From Review – TomazicM Jun 21 '19 at 11:42
2

Like this? (not tested).

(1) Merging all fields of 12 layers

SELECT * FROM "_January"
UNION ALL
SELECT * FROM "_February"
UNION ALL
............. // please repeat - March to November
UNION ALL
SELECT * FROM "_December"

(2) Merging layers, and selecting only TEST field

SELECT TEST, "_January".geometry FROM "_January"
UNION ALL
SELECT TEST, "_February".geometry FROM "_February"
UNION ALL
............ // please repeat  - March to November
UNION ALL
SELECT TEST, "_December.geometry" FROM "_December"

UPDATE (3) Using table from (2), choose only TEST field is TRUE

SELECT a.TEST, a.geometry AS geometry
FROM (
     SELECT TEST, "_January".geometry FROM "_January"
     UNION ALL
     SELECT TEST, "_February".geometry FROM "_February"
     UNION ALL
     ............ // please repeat  - March to November
     UNION ALL
     SELECT TEST, "_December".geometry FROM "_December"
) AS a
WHERE TEST = 'TRUE'
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  • work, thanks! I take advantage of it, instead I wanted to select from the TEST column the TRUE values, how can I modify the sql formula? – Fabrizio Jun 14 '19 at 13:31
  • hi @Fabrizio How about add another filter on top of the expression written in the above example (2)? I mean we can put the output of (2) as a table, and set SELECT syntax from this new table. – Kazuhito Jun 14 '19 at 13:38
  • hi again @Fabrizio Not sure if it works, but I tried to add (3) which selects only if TEST field is TRUE. Let me know if I have misunderstood the request. – Kazuhito Jun 14 '19 at 13:54

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