GROUP_CONCAT（MySQL）和STRING_AGG（SQL Server、PostgreSQL等）用于將多行數(shù)據(jù)合并為單個字符串。1. 它們通過將同一組的多個值合并到一個字段中來避免重復；2. 可自定義分隔符，如使用分號或空格代替逗號；3. 支持對聚合值排序以提高可讀性；4. 可過濾重復值以保持輸出簡潔。例如，在用戶及其顏色數(shù)據(jù)中，它們能將多個顏色合并為“red, blue”等形式，適用于報表、儀表板和數(shù)據(jù)導出場景。

GROUP_CONCAT (in MySQL) or STRING_AGG (in SQL Server, PostgreSQL, etc.) is used to combine multiple rows of data into a single string, which is especially helpful when you want to see all related values in one field instead of separate rows.

For example, imagine you have a table that lists people and their hobbies. Instead of seeing each hobby on a separate line for the same person, you can use GROUP_CONCAT or STRING_AGG to show them all in one row like: "reading, cooking, hiking".

Combine Related Rows into One String

This function helps avoid repetition by merging multiple values from the same group.
Say you're querying a database with user IDs and their favorite colors:

Using GROUP_CONCAT or STRING_AGG would give you:

• In MySQL:

  SELECT user_id, GROUP_CONCAT(color) AS colors
  FROM users
  GROUP BY user_id;

• In SQL Server or PostgreSQL:

  SELECT user_id, STRING_AGG(color, ', ') AS colors
  FROM users
  GROUP BY user_id;

You get a much cleaner view without repeating the same user ID across multiple rows.

Customize the Separator and Order

By default, commas are used as separators, but you can change that. For example, if you want to separate values with a semicolon or even a space:

• MySQL:

  SELECT user_id, GROUP_CONCAT(color SEPARATOR '; ') AS colors
  FROM users
  GROUP BY user_id;

• SQL Server / PostgreSQL:

  SELECT user_id, STRING_AGG(color, '; ') AS colors
  FROM users
  GROUP BY user_id;

Also, you can sort the values inside the aggregated string:

• MySQL:

  SELECT user_id, GROUP_CONCAT(color ORDER BY color) AS colors
  FROM users
  GROUP BY user_id;

• SQL Server / PostgreSQL:

  SELECT user_id, STRING_AGG(color, ', ' WITHIN GROUP (ORDER BY color)) AS colors
  FROM users
  GROUP BY user_id;

Sorting makes the result more predictable and easier to read, especially when dealing with alphabetical or time-based data.

Handle Duplicate Values

If your dataset has duplicates and you only want unique values, most versions allow filtering those out:

• In MySQL, you can add DISTINCT directly:

  SELECT user_id, GROUP_CONCAT(DISTINCT color) AS colors
  FROM users
  GROUP BY user_id;

• In SQL Server or PostgreSQL, you’d deduplicate before aggregating:

  SELECT user_id, STRING_AGG(color, ', ')
  FROM (
    SELECT DISTINCT user_id, color
    FROM users
  ) AS deduped
  GROUP BY user_id;

This helps keep your output clean and avoids cluttering the string with repeated values.

In short, GROUP_CONCAT and STRING_AGG are powerful tools when you need to flatten grouped data into readable strings. They’re commonly used in reporting, dashboard views, and when preparing data for export or display. Just remember to watch for long strings, duplicate values, and separator consistency — and you’ll be good to go.基本上就這些。

以上是SQL中使用的group_concat（或string_agg）函數(shù)是什么？的詳細內(nèi)容。更多信息請關注PHP中文網(wǎng)其他相關文章！