MySQL-仅在具有值的情况下选择列

Question

我需要构建一个MySQL查询，其中仅当它具有给定的4个列first_accuracy ， second_accuracy ， third_accuracy ， fourth_accuracy具有值（不是NOT NULL ）时才需要选择列。

假设我只有first_accuracy ， second_accuracy的值，那么查询应该是

从Compliance_employees CE中选择first_accuracy，second_accuracy。

更新：

我套用的查询：

SELECT first_accuracy, second_accuracy, third_accuracy, fourth_accuracy FROM compliance_employees CE;

结果：

您可能会看到，对于“第三精度”，任何行都没有值，因此我根本不想选择该列。 那可能吗？

有什么办法吗？

Answer 1

如果您只想确保两列first_accuracy和second_accuracy不为NULL使用以下命令：

SELECT first_accuracy,
       second_accuracy
FROM compliance_employees CE
WHERE first_accuracy  IS NOT NULL AND
      second_accuracy IS NOT NULL

如果要确保所有四个列都不为NULL使用以下命令：

SELECT first_accuracy,
       second_accuracy
FROM compliance_employees CE
WHERE first_accuracy  IS NOT NULL AND
      second_accuracy IS NOT NULL AND
      third_accuracy  IS NOT NULL AND
      fourth_accuracy IS NOT NULL

Answer 2

使用'UNION'，GROUP_CONCAT（）和'not null'，我们找到具有值的列数。 然后将列传递给“准备的陈述”，从而为您提供所需的结果。

        select coalesce (GROUP_CONCAT(col_name),'No_Result') into @ColumnNames
        from (
        select 'first_accuracy' as col_name, count( distinct first_accuracy) as count from compliance_employees
        where first_accuracy  is not null 
        union
        select 'second_accuracy' as col_name, count( distinct second_accuracy) from compliance_employees
        where second_accuracy  is not null
        union
        select 'third_accuracy' as col_name, count( distinct third_accuracy) from compliance_employees
        where third_accuracy  is not null
        union
        select 'fourth_accuracy' as col_name, count( distinct fourth_accuracy) from compliance_employees
        where fourth_accuracy is not null
        )a where count >0\\
        -- select @ColumnNames\\


    SET @sql := CONCAT('SELECT ', @ColumnNames, ' FROM compliance_employees');
    set @sql := REPLACE(@sql,'No_Result','"No Column have values" as No_Result')\\

     PREPARE stmt FROM @sql;
     EXECUTE stmt;          

在此处检查Live demo。

输出：

Answer 3

SELECT 
    first_accuracy, 
    second_accuracy
FROM
    compliance_employees CE
WHERE 
    first_accuracy IS NOT NULL
    AND second_accuracy IS NOT NULL

Answer 4

您是否要问如何从四个字段中选择值？ 如果是这样，我建议使用GREATEST ：

SELECT 
    GREATEST(first_accuracy, second_accuracy) as accuracy
FROM
    compliance_employees CE;