Question

在子查询上使用GROUP_CONCAT时，我有一种奇怪的行为。这是我的疑问：

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;

（这是一个“测试”查询，可以轻松显示问题，真正的查询不是“哑”）。它重新组合两个子查询的结果。所有ID都存在并返回一行。

因此，当我单独运行第一个子查询时，我得到名称的结果“APN”，并且“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（完整ID列表）

第二个子查询是相同的，除了名称是“智能”且ID不同。所以这是预期的行为。

问题是当我运行完整查询时，对于名称APN，我得到以下ID列表： 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

所以这个列表比第一个小得多。这个名称“聪明”也是如此。

我尝试用（SELECT“APN”作为名称，“1-8-9-10-11-12-14-17-etc ...”替换我的两个子查询作为id FROM site LIMIT 1） ID列表（以及名称“smart”的相同），以及完整查询的结果与预期一致（每个名称的完整ID列表）。

我的服务器上的group_concat_max_len是1024（我的完整ID列表比1024个字符小得多）

那么，您是否知道为什么结果不如预期？

Answer 1

您的查询有点奇怪。

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;

等于：

    (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))

除非您的原始查询产生许多具有相同ID组的APN行，否则无需按名称和ID进行父分组。

回到你的问题：你是正确的，group_concat的最大长度为1024，但排序/联合操作进一步截断为1/3（1024/3 = 341）。（虽然它已知，但没有官方文件可供支持）

在您的情况下，只需增加组concat max length值：

SET group_concat_max_len = 5000;

并且应该在不截断的情况下提供所需的输出。

您可以创建临时表并将它们联合起来，也可以将grou_concat结果输出到变量中。在两种方式中，grop_concat都将截断其原始默认值。

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