Question

在MySQL中，我有三个表格如下：

USERS：

USER_ID | USERNAME
1       | Homer Simpson

字段：

FIELD_ID | FIELD_NAME
1        | Phone
2        | Address

FIELD_VALUES：

USER_ID | FIELD_ID | FIELD_VALUE
1       | 1        | 555-1212
1       | 2        | 10 biteme drive

我想要做的是组合三个表，以便每个用户最终得到一行，字段名称为列标题和字段值，所以我最终得到：

USERS_TABLE：

USER_ID  | USER_NAME     | PHONE    | ADDRESS
1        | Homer Simpson | 555-1212 | 10 biteme drive

Answer 1

如果字段名称（电话和地址）已修复，您可以使用此技巧来转移数据：

SELECT
  users.user_id,
  users.user_name,
  MAX(CASE WHEN field_id=1 THEN field_value END) AS Phone,
  MAX(CASE WHEN field_id=2 THEN field_value END) AS Address
FROM
  users INNER JOIN field_values
  ON users.user_id = field_values.user_id
GROUP BY
  users.user_id,
  users.user_name

但如果您的值未修复，则需要使用动态查询：

SELECT    
  CONCAT(    
    'SELECT users.user_id,users.username,',    
    GROUP_CONCAT(CONCAT('MAX(CASE WHEN field_id=', field_id, ' THEN field_value END) AS `', field_name, '`')),    
    ' FROM users INNER JOIN field_values ON users.user_id = field_values.user_id',
    ' GROUP BY users.user_id, users.username')    
FROM `fields` INTO @sql;

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

请参阅小提琴here。

Answer 2

尝试以下方法： - SELECT users.user_id,username, fv1.field_value value1, innertab.field_value value2 FROM USERS LEFT JOIN FIELD_VALUES fv1 ON users.user_id = fv1.user_id INNER JOIN FIELDS f1 ON fv1.FIELD_ID = f1.FIELD_ID AND f1.FIELD_ID = 1 LEFT JOIN (SELECT users.user_id, FIELD_VALUES.field_value FROM USERS LEFT JOIN FIELD_VALUES ON users.user_id = FIELD_VALUES.user_id INNER JOIN FIELDS ON FIELD_VALUES.FIELD_ID = FIELDS.FIELD_ID AND FIELDS.FIELD_ID = 2) AS innertab ON innertab.user_id = users.user_id

sql fiddle

用于组合和展平表的SQL

2 个答案: