SQLite：尝试以比 UNION 字符串更优雅的方式连接列

Posted 2023-02-24

【中文标题】SQLite：尝试以比 UNION 字符串更优雅的方式连接列

【英文标题】：SQLite: Trying to concatenate columns in more elegant way than string of UNIONs

【发布时间】：2014-08-13 21:44:09

【问题描述】：

我有一个包含以下字段和示例值的表：

coursecode  BIO 101
a_code      FA
b_code      SP
c_code      SU
d_code
e_code

我正在尝试用逗号分隔的所有具有值的“代码”字段（即“FA、SP、SU”）的列表填充另一个表中的字段（使用 coursecode 作为键）。我用下面的代码成功地完成了这个，但它很混乱，想知道是否有更优雅的方式来完成这个（特别是没有所有 UNION 语句）：

SELECT coursecode, group_concat(codes)
FROM (
SELECT 
    coursecode, 
    a_code AS codes
FROM scbsupp
    WHERE coursecode="BIO 317"
UNION
SELECT coursecode,
    b_code AS codes
FROM scbsupp
    WHERE coursecode="BIO 317" 
UNION
SELECT 
    coursecode, 
    c_code AS codes
FROM scbsupp
    WHERE coursecode="BIO 317"
UNION
SELECT coursecode,
    d_code AS codes
FROM scbsupp
    WHERE coursecode="BIO 317"
UNION
SELECT coursecode,
    e_code AS codes 
FROM scbsupp
    WHERE coursecode="BIO 317"
) WHERE codes != "";    

根据我自己的知识，我想知道是否有更好的方法来做到这一点，感谢任何帮助！

【参考方案1】：

你可以这样做：

SELECT coursecode,
      LTRIM(
              CASE WHEN (a_code IS NOT NULL AND a_code <> '') THEN        a_code ELSE '' END ||
              CASE WHEN (b_code is NOT NULL AND b_code <> '') THEN ',' || b_code ELSE '' END ||
              CASE WHEN (c_code IS NOT NULL AND c_code <> '') THEN ',' || c_code ELSE '' END ||
              CASE WHEN (d_code IS NOT NULL AND d_code <> '') THEN ',' || d_code ELSE '' END ||
              CASE WHEN (e_code IS NOT NULL AND e_code <> '') THEN ',' || e_code ELSE '' END, ','
            )   AS code
  FROM scbsupp
    WHERE coursecode="BIO 317"

;