Question

这是我当前的表格

╔══════╦═══╦═══╦═══╦═════╦════╗
║ YEAR ║ A ║ B ║ C ║ ... ║ ZZ ║
╠══════╬═══╬═══╬═══╬═════╬════╣
║ 1995 ║ 1 ║ 0 ║ 1 ║ ... ║  1 ║
║ 1996 ║ 1 ║ 1 ║ 0 ║ ... ║  0 ║
╚══════╩═══╩═══╩═══╩═════╩════╝

该表包含大约1000列。现在我需要编写一个代码来将这个表旋转到一个更合理的表中，其中我表示的值表示为1.所以结果表应该像

╔══════╦════════╦═══════╗
║ YEAR ║ COLUMN ║ VALUE ║
╠══════╬════════╬═══════╣
║ 1995 ║ A      ║     1 ║
║ 1995 ║ C      ║     1 ║
║ 1995 ║ ZZ     ║     1 ║
║ 1996 ║ A      ║     1 ║
║ 1996 ║ B      ║     1 ║
╚══════╩════════╩═══════╝

所以通过谷歌搜索，我开始创建一个如下的SQL

BEGIN
    DECLARE v_finished INTEGER DEFAULT 0;
    DECLARE v_du **???** DEFAULT "";

    DEClARE du_cursor CURSOR FOR SELECT * FROM date_usage;

    DECLARE CONTINUE HANDLER FOR NOT FOUND SET v_finished = 1;

    OPEN du_cursor;

    get_du: LOOP

        FETCH du_cursor INTO v_du;

        IF v_finished = 1 THEN 
            LEAVE get_du;
        END IF;

        SHOW v_du; <<---

    END LOOP get_du;

    CLOSE du_cursor; 
END

但问题是我不确定如何将v_du指定为列，我不确定我将如何读取列名。我们将不胜感激。

Thankx

Answer 1

最简单的方法是使用一堆union all语句：

select year, 'A' as column, A as value from table where A > 0 union all
select year, 'B' as column, B as value from table where B > 0 union all
. . .
select year, 'ZZ' as column, ZZ as value from table where ZZ > 0;

我会使用information_schema.columns列出列，并使用Excel生成代码。

对大型表格使用效率更高的方法是cross join：

select year, col,
       (case when col = 'A' then A
             when col = 'B' then B
             . . .
             when col = 'ZZ' then ZZ
        end) as value
from table t cross join
     (select 'A' as col union all select 'B' union all . . . select 'ZZ') cols
having value > 0;

Answer 2

如果要将数据迁移到新表，请按以下方式执行：

INSERT INTO new_table(`year`, `column`, `value`) SELECT year, 'A', A FROM oldTable WHERE A > 0;
INSERT INTO new_table(`year`, `column`, `value`) SELECT year, 'B', B FROM oldTable WHERE B > 0;
INSERT INTO new_table(`year`, `column`, `value`) SELECT year, 'C', C FROM oldTable WHERE C > 0;

重复所有列名称。

如何从MySql旋转表

2 个答案: