python - 我应该对派生表使用 LEFT JOIN、RIGHT JOIN、不同的表设计还是需要编码？

Question

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Answer 1

SELECT MAX(CASE WHEN year = 2005 THEN Symbol ELSE NULL END) `2005`,
       MAX(CASE WHEN year = 2006 THEN Symbol ELSE NULL END) `2006`,
       MAX(CASE WHEN year = 2007 THEN Symbol ELSE NULL END) `2007`
FROM tableName
GROUP BY Symbol

• SQLFiddle Demo

如果您有未知的年份值，动态 SQL 查询是首选。

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'MAX(CASE WHEN year = ',
      year,
      ' THEN Symbol ELSE NULL end) AS ',
      CONCAT('`', year, '`')
    )
  ) INTO @sql
FROM TableName;

SET @sql = CONCAT('SELECT  ', @sql, ' 
                   FROM tableName 
                   GROUP BY Symbol');

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

• SQLFiddle Demo

如果您想显示空字符串而不是 null 值，则需要 COALESCE。

• SQLFiddle Demo
• SQLFiddle Demo (Dynamic Query)