Weird result for GROUP_CONCAT on subquery

I am having a weird behavior when using GROUP_CONCAT on subquery. Here is my query :

SELECT
  name,
  GROUP_CONCAT(DISTINCT (id) SEPARATOR "-") AS id
FROM (
       (SELECT
          "APN"                                          AS name,
          GROUP_CONCAT(DISTINCT (site.id) SEPARATOR "-") AS id
        FROM site
        WHERE id IN
              (138, 147, 8918, 8916, 9033, 9240, 97, 9038, 8886, 9036, 9067, 146, 37, 9127, 52, 9031, 23, 8635, 8665, 
               46, 39, 18, 33, 9035, 137, 9051, 8766, 25, 20, 9160, 133, 8636, 9021, 8655, 21, 42, 8757, 22, 9017, 77, 
               9037, 44, 49, 9323, 55, 74, 150, 8, 67, 1, 8928, 58, 9025, 9221, 9019, 9069, 9214, 9176, 95, 40, 9335, 
               168, 9260, 8641, 9227, 9258, 24, 50, 29, 9073, 12, 36, 8882, 9, 43, 76, 9032, 51, 9060, 96, 8922, 9212,
               14, 9095, 28, 9213, 31, 41, 68, 9027, 8884, 9023, 9059, 9034, 9016, 11, 61, 9229, 8761, 9225, 8937, 9018,
               9121, 9119, 8659, 8926, 9096, 57, 9083, 8662, 9232, 149, 8643, 88, 19, 8660, 10, 8936, 9210, 9241, 17, 8872))
       UNION ALL
       (SELECT
          "smart"                                        AS name,
          GROUP_CONCAT(DISTINCT (site.id) SEPARATOR "-") AS id
        FROM site
        WHERE id IN
              (9129, 8981, 9136, 9169, 9170, 9171, 9172, 9297, 9147, 9155, 9139, 9138, 9142, 9296, 8987, 9216, 9252,
               9320, 8951, 8945, 8952, 8965, 8963, 9012, 9192, 8938, 8941, 8968, 8977, 9117, 9135, 9140, 9143, 9295,
               9298, 9137, 8988, 8989, 8992, 9164, 9156, 9165, 9168, 9173, 8953, 8999, 8939, 8940, 8942, 8943, 8954,
               8956, 8957, 8959, 8960, 8964, 8971, 8972, 8973, 8974, 8982, 9000, 9001, 9003, 8950, 8978, 8979, 8983,
               9002, 9005, 8984, 8955, 8986, 8980, 8993, 9008, 9010, 8949, 8998, 9150, 9122, 8944, 8946, 8948, 9006,
               9009, 9013, 9128, 9215, 9321, 9011, 9154, 8970, 8975, 8994, 9070, 8966, 8958, 9007, 9014))
     ) t
GROUP BY name;

(This is a "test" query to show easily the issue, the real query is not that "dumb"). It regroups the result of two subqueries. All the IDs exist and return a row.

So when I run the first subquery alone, I get the result "APN" for name, and " 1-8-9-10-11-12-14-17-18-19-20-21-22-23-24-25-28-29-31-33-36-37-39-40-41-42-43-44-46-49-50-51-52-55-57-58-61-67-68-74-76-77-88-95-96-97-133-137-138-146-147-149-150-168-8635-8636-8641-8643-8655-8659-8660-8662-8665-8757-8761-8766-8872-8882-8884-8886-8916-8918-8922-8926-8928-8936-8937-9016-9017-9018-9019-9021-9023-9025-9027-9031-9032-9033-9034-9035-9036-9037-9038-9051-9059-9060-9067-9069-9073-9083-9095-9096-9119-9121-9127-9160-9176-9210-9212-9213-9214-9221-9225-9227-9229-9232-9240-9241-9258-9260-9323-9335" for ID (the full list of IDs)

It is the same for the second subquery, except that the name is "smart" and the IDs are different. So this is the expected behavior.

The issue is when I run the complete query, for the name APN, I get the following list of IDs : 1-8-9-10-11-12-14-17-18-19-20-21-22-23-24-25-28-29-31-33-36-37-39-40-41-42-43-44-46-49-50-51-52-55-57-58-61-67-68-74-76-77-88-95-96-97-133-137-138-146-147-149-150-168-8635-8636-8641-8643-8655-8659-8660-8662-8665-8757-8761-8766-8872-8882-8884-8886-8916-8918-8922-8926-8928-8936-8937-9016-9017-9018-9019-9021-9023-9025-9027-9031-9032-9033-9034

So this list is much smaller than the first one. And this is the same for the name "smart".

I tried replacing my two subqueries by (SELECT "APN" as name, "1-8-9-10-11-12-14-17-etc..." as id FROM site LIMIT 1) with the complete list of IDs (and the same for the name "smart"), and with that, the result of the full query is as expected (the full list of ID for each name).

The group_concat_max_len is 1024 on my server (and my full ID list are much more smaller than 1024 caracters)

So, do you have any idea why the result is not as expected ?

Your query is bit weird.

select name, GROUP_CONCAT(DISTINCT(id) SEPARATOR "-") AS id FROM (
        (select "APN" AS name, GROUP_CONCAT(DISTINCT(site.id) SEPARATOR "-") AS id  from site WHERE id IN (138,147,8918,8916,9033,9240,97,9038,8886,9036,9067,146,37,9127,52,9031,23,8635,8665,46,39,18,33,9035,137,9051,8766,25,20,9160,133,8636,9021,8655,21,42,8757,22,9017,77,9037,44,49,9323,55,74,150,8,67,1,8928,58,9025,9221,9019,9069,9214,9176,95,40,9335,168,9260,8641,9227,9258,24,50,29,9073,12,36,8882,9,43,76,9032,51,9060,96,8922,9212,14,9095,28,9213,31,41,68,9027,8884,9023,9059,9034,9016,11,61,9229,8761,9225,8937,9018,9121,9119,8659,8926,9096,57,9083,8662,9232,149,8643,88,19,8660,10,8936,9210,9241,17,8872)) 
    UNION ALL 
        (select "smart" AS name, GROUP_CONCAT(DISTINCT(site.id) SEPARATOR "-") AS id from site WHERE id IN (9129,8981,9136,9169,9170,9171,9172,9297,9147,9155,9139,9138,9142,9296,8987,9216,9252,9320,8951,8945,8952,8965,8963,9012,9192,8938,8941,8968,8977,9117,9135,9140,9143,9295,9298,9137,8988,8989,8992,9164,9156,9165,9168,9173,8953,8999,8939,8940,8942,8943,8954,8956,8957,8959,8960,8964,8971,8972,8973,8974,8982,9000,9001,9003,8950,8978,8979,8983,9002,9005,8984,8955,8986,8980,8993,9008,9010,8949,8998,9150,9122,8944,8946,8948,9006,9009,9013,9128,9215,9321,9011,9154,8970,8975,8994,9070,8966,8958,9007,9014))
) t GROUP BY name;

is equal to:

    (select "APN" AS name, GROUP_CONCAT(DISTINCT(site.id) SEPARATOR "-") AS id  from site WHERE id IN (138,147,8918,8916,9033,9240,97,9038,8886,9036,9067,146,37,9127,52,9031,23,8635,8665,46,39,18,33,9035,137,9051,8766,25,20,9160,133,8636,9021,8655,21,42,8757,22,9017,77,9037,44,49,9323,55,74,150,8,67,1,8928,58,9025,9221,9019,9069,9214,9176,95,40,9335,168,9260,8641,9227,9258,24,50,29,9073,12,36,8882,9,43,76,9032,51,9060,96,8922,9212,14,9095,28,9213,31,41,68,9027,8884,9023,9059,9034,9016,11,61,9229,8761,9225,8937,9018,9121,9119,8659,8926,9096,57,9083,8662,9232,149,8643,88,19,8660,10,8936,9210,9241,17,8872)) 
    UNION ALL 
    (select "smart" AS name, GROUP_CONCAT(DISTINCT(site.id) SEPARATOR "-") AS id from site WHERE id IN (9129,8981,9136,9169,9170,9171,9172,9297,9147,9155,9139,9138,9142,9296,8987,9216,9252,9320,8951,8945,8952,8965,8963,9012,9192,8938,8941,8968,8977,9117,9135,9140,9143,9295,9298,9137,8988,8989,8992,9164,9156,9165,9168,9173,8953,8999,8939,8940,8942,8943,8954,8956,8957,8959,8960,8964,8971,8972,8973,8974,8982,9000,9001,9003,8950,8978,8979,8983,9002,9005,8984,8955,8986,8980,8993,9008,9010,8949,8998,9150,9122,8944,8946,8948,9006,9009,9013,9128,9215,9321,9011,9154,8970,8975,8994,9070,8966,8958,9007,9014))

No need for parent grouping by name and id unless your original query produces many APN Rows with same group of ID's.

back to your question: You are correct that group_concat has max length of 1024 but a sort /union operation truncates further to 1/3 (1024/3 = 341). (although it's known but no official document is available to back this up)

In your case, just increase the group concat max length value:

SET group_concat_max_len = 5000;

and that should give your desired output without truncating.

You can create temporary tables and union them, or you can output the grou_concat result into a variable. In both way grop_concat will truncate by its original default value.