我可以将多个MySQL行连接到一个字段中吗? [英] Can I concatenate multiple MySQL rows into one field?

问题描述

使用MySQL，我可以执行以下操作:

Using MySQL, I can do something like:

SELECT hobbies FROM peoples_hobbies WHERE person_id = 5;

我的输出:

shopping
fishing
coding

但是我只想要1行1列:

but instead I just want 1 row, 1 col:

预期输出:

shopping, fishing, coding

原因是我要从多个表中选择多个值，并且在所有联接之后，我得到的行比我想要的要多得多.

The reason is that I'm selecting multiple values from multiple tables, and after all the joins I've got a lot more rows than I'd like.

我在 MySQL Doc ，它看起来不像CONCAT或CONCAT_WS函数接受结果集.

I've looked for a function on MySQL Doc and it doesn't look like the CONCAT or CONCAT_WS functions accept result sets.

那么这里有人知道怎么做吗?

So does anyone here know how to do this?

推荐答案

您可以使用

You can use GROUP_CONCAT:

SELECT person_id, GROUP_CONCAT(hobbies SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;

如路德维希在他的评论中所述，您可以添加DISTINCT运算符以避免重复:

As Ludwig stated in his comment, you can add the DISTINCT operator to avoid duplicates:

SELECT person_id, GROUP_CONCAT(DISTINCT hobbies SEPARATOR ', ')
FROM peoples_hobbies 
GROUP BY person_id;

如Jan在他们的评论中所述，您也可以在使用ORDER BY将其内插之前对值进行排序:

As Jan stated in their comment, you can also sort the values before imploding it using ORDER BY:

SELECT person_id, GROUP_CONCAT(hobbies ORDER BY hobbies ASC SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;

如达格在他的评论中所述， 结果限制为1024个字节.要解决此问题，请在查询之前运行以下查询:

As Dag stated in his comment, there is a 1024 byte limit on the result. To solve this, run this query before your query:

SET group_concat_max_len = 2048;

当然，您可以根据需要更改2048.要计算和分配值，请执行以下操作:

Of course, you can change 2048 according to your needs. To calculate and assign the value:

SET group_concat_max_len = CAST(
    (SELECT SUM(LENGTH(hobbies)) + COUNT(*) * LENGTH(', ')
    FROM peoples_hobbies 
    GROUP BY person_id)
    AS UNSIGNED
);

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