嵌套查询中的 COALESCE

Question

尝试阻止此查询在结果（在本例中为 lessons.classDate）不存在时返回 NULL。我确定 COALESCE 应该被环绕，但我无法正确理解语法。

SELECT FLOOR(COUNT(S0001)* 100 / 
        (SELECT COUNT(*) FROM stat_P01627
        INNER JOIN lessons ON stat_P01627.lesson = lessons.id 
        INNER JOIN modules ON lessons.module = modules.id 
        WHERE lessons.module = 'MHG405294' AND lessons.classDate >= DATE_ADD(CURDATE(),INTERVAL -12 WEEK))
        ) AS stat12wkMod1 
        FROM attendance_P01627
        INNER JOIN lessons ON stat_P01627.lesson = lessons.id 
        INNER JOIN modules ON lessons.module = modules.id 
        WHERE lessons.module = 'MHG405294' AND lessons.classDate >= DATE_ADD(CURDATE(),INTERVAL -12 WEEK);

我在努力；

SELECT FLOOR(COUNT(S0001)* 100 / 
        COALESCE(SELECT COUNT(*) FROM stat_P01627
        INNER JOIN lessons ON stat_P01627.lesson = lessons.id 
        INNER JOIN modules ON lessons.module = modules.id 
        WHERE lessons.module = 'MHG405294' AND lessons.classDate >= DATE_ADD(CURDATE(),INTERVAL -12 WEEK)),0)
        ) AS stat12wkMod1 
        FROM stat_P01627
        INNER JOIN lessons ON stat_P01627.lesson = lessons.id 
        INNER JOIN modules ON lessons.module = modules.id 
        WHERE lessons.module = 'MHG405294' AND lessons.classDate >= DATE_ADD(CURDATE(),INTERVAL -12 WEEK);

[编辑] 工作版本，供感兴趣的人使用。

SELECT IFNULL(FLOOR(COUNT(S0001)* 100 / 
    IFNULL((SELECT COUNT(*) FROM stat_P01627
    INNER JOIN lessons ON stat_P01627.lesson = lessons.id 
    INNER JOIN modules ON lessons.module = modules.id 
    WHERE lessons.module = 'MHG405294' AND lessons.classDate >= DATE_ADD(GETDATE(),INTERVAL -12 WEEK)),1)) ,1)
    AS stat12wkMod1 
    FROM stat_P01627
    INNER JOIN lessons ON stat_P01627.lesson = lessons.id 
    INNER JOIN modules ON lessons.module = modules.id 
    WHERE lessons.module = 'MHG405294' AND lessons.classDate >= DATE_ADD(GETDATE(),INTERVAL -12 WEEK);

Answer 1

对 SQL-SERVER 使用 Isnull，对 MySQL

使用 IFNULL

SELECT FLOOR(COUNT(S0001)* 100 / 
        IFNULL((SELECT COUNT(*) FROM stat_P01627
        INNER JOIN lessons ON stat_P01627.lesson = lessons.id 
        INNER JOIN modules ON lessons.module = modules.id 
        WHERE lessons.module = 'MHG405294' AND lessons.classDate >= DATE_ADD(CURDATE(),INTERVAL -12 WEEK)),1)
        ) AS stat12wkMod1 
        FROM stat_P01627
        INNER JOIN lessons ON stat_P01627.lesson = lessons.id 
        INNER JOIN modules ON lessons.module = modules.id 
        WHERE lessons.module = 'MHG405294' AND lessons.classDate >= DATE_ADD(CURDATE(),INTERVAL -12 WEEK);

https://docs.microsoft.com/en-us/sql/t-sql/language-elements/coalesce-transact-sql

The ISNULL OR IFNULL function and the COALESCE expression have a similar purpose but can behave differently.

Because ISNULL is a function, it is evaluated only once.

As described above, the input values for the COALESCE expression can be evaluated multiple times. Data type determination of the resulting expression is different. ISNULL uses the data type of the first parameter, COALESCE follows the CASE expression rules and returns the data type of value with the highest precedence.

The NULLability of the result expression is different for ISNULL and COALESCE. The ISNULL return value is always considered NOT NULLable (assuming the return value is a non-nullable one) whereas COALESCE with non-null parameters is considered to be NULL. So the expressions ISNULL(NULL, 1) and COALESCE(NULL, 1) although equivalent have different nullability values. This makes a difference if you are using these expressions in computed columns, creating key constraints or making the return value of a scalar UDF deterministic so that it can be indexed as shown in the following example.

嵌套查询中的 COALESCE

COALESCE in Nested Query

sql

coalesce