MySQL视图：在另一个计算字段中引用一个计算字段（按名称）

Question

我如何定义一个具有两个计算字段的视图，例如…

 ('TableName'.'BlueSquares' + 'TableName'.'RedSquares') AS TotalSquares, ('TableName'.'BlueCirles' + 'TableName'.'RedCircles') AS TotalCircles

…并根据前两个计算字段创建第三个计算字段，如…

 ('ViewName'.'TotalSquares' + 'ViewName'.'TotalCircles') AS TotalShapes

…？

当我按名称引用前两个计算字段时，我收到一条消息，指出字段未知。

谢谢！

Answer 1

由于视图中不允许子查询，因此您需要通过创建多个视图来模拟它们。

例如，如果直接执行此查询，将解决您的问题：

SELECT 
    TotalCircles + TotalSquares AS TotalShapes
FROM
    (SELECT 
        BlueCirles + RedCircles AS TotalCircles,
        BlueSquares + RedSquares AS TotalSquares
    FROM
        (SELECT
            2 AS BlueCirles,
            3 AS RedCircles,
            4 AS BlueSquares,
            5 AS RedSquares
        ) AS shapes
    ) as totals;

根据MySQL文档，视图在FROM子句中具有不能包含子查询的限制。要变通解决此限制并将该查询转换为视图，请将其分解为3个视图（每个子查询一个），最后一个给出所需的字段组合：

CREATE VIEW shapes AS
SELECT
    2 AS BlueCirles,
    3 AS RedCircles,
    4 AS BlueSquares,
    5 AS RedSquares;

CREATE VIEW totals AS
SELECT 
    BlueCirles + RedCircles AS TotalCircles,
    BlueSquares + RedSquares AS TotalSquares
FROM
    shapes;

CREATE VIEW result AS
SELECT 
    TotalCircles + TotalSquares AS TotalShapes
FROM
    totals;

SELECT * FROM result;