Question

如果value_two中的值null不存在（或table_two}，我希望默认使用value_two中的值table_one。< / p>

我有以下查询格式：

SELECT one.value_one, two.value_two
    FROM schema.table_one one 
    LEFT JOIN schema.table_two two 
        ON one.id_value = two.id_value and two.other_column = 'other_val'
    WHERE one.id_value = 'id_val';

我知道一个选项是选择所有值并在返回以下内容后在客户端处理：

SELECT one.value_one, one.value_two, two.value_two as two_value_two
    FROM schema.table_one one 
    LEFT JOIN schema.table_two two 
        ON one.id_value = two.id_value and two.other_column = 'other_val'
    WHERE one.id_value = 'id_val';

检查two.value_two是否存在“干净”方式，如果不存在则默认选择one.value_two

Answer 1

您可以使用COALESCE功能。

COALESCE返回非空的第一个参数。

像这样使用：

SELECT COALESCE(two.value_two, one.value_two)
FROM schema.table_one one 
LEFT JOIN schema.table_two two ON one.id_value = two.id_value and two.other_column = 'other_val' 
WHERE one.id_value = 'id_val';

请注意COALESCE是一个ANSI SQL函数，因此可用于SQL的所有实现（MySQL，MS SQL Server，...）

Answer 2

angularjs

https://www.postgresql.org/docs/current/static/functions-conditional.html#FUNCTIONS-COALESCE-NVL-IFNULL

从存在值的表中选择？

2 个答案: