Question

我的头痛已经开始，因为我试图制作这个MySQL视图，其中一个列的结果值为NULL。我需要有一个像0这样的实际值。

我从table1中获取我的id并在table2中对它进行比较，所以它不确定是否有一个带有该id的数字，如果table2中没有数字，则该值将变为NULL，而我需要0。这是我的视图代码：

CREATE VIEW `instock` AS
SELECT 
    table1.name AS name,
    table1.supl AS supply,
    table2.num AS numbers,
    table1.maxnum AS maxnumbers
FROM
    (table1
    LEFT JOIN table2 ON ((table1.id = table2.id)))
ORDER BY table1.name

它的列号，其中我有一个NULL值

Answer 1

您可以使用 ifnull 功能：

SELECT 
  table1.name AS name,
  table1.supl AS supply,
  ifnull(table2.num,0) AS numbers,
  table1.maxnum AS maxnumbers
FROM
  (table1
  LEFT JOIN table2 ON ((table1.id = table2.id)))
ORDER BY table1.name

Answer 2

我不确定查询中的left join。 left join从 left 表（table1）中提供所有行，包括那些在其他表中没有对应的行（{{1} }）。对于右表中这些不匹配的行，您将获得所有table2列的NULL，包括table2。

或许您正在寻找table2.num。这取决于您的数据以及您的inner join是否为NULL。要仅使用零替换NULL，请使用COALESCE

table2.num

或IFNULL

CREATE VIEW `instock` AS
SELECT 
    table1.name AS name,
    table1.supl AS supply,
    COALESCE(table2.num,0) AS numbers,
    table1.maxnum AS maxnumbers
FROM
    (table1
    LEFT JOIN table2 ON ((table1.id = table2.id)))
ORDER BY table1.name

如果您想完全跳过CREATE VIEW `instock` AS SELECT table1.name AS name, table1.supl AS supply, IFNULL(table2.num,0) AS numbers, table1.maxnum AS maxnumbers FROM (table1 LEFT JOIN table2 ON ((table1.id = table2.id))) ORDER BY table1.name中不在table2内的项目（与ID字段相对应），您可以使用table1：

inner join

再次：这取决于您的需求。 IFNULL / COALESCE将显示CREATE VIEW `instock` AS SELECT table1.name AS name, table1.supl AS supply, table2.num AS numbers, table1.maxnum AS maxnumbers FROM (table1 INNER JOIN table2 ON ((table1.id = table2.id))) ORDER BY table1.name而不是NULL，INNER JOIN将完全跳过这些行。

（如果有疑问，我总是提到this explanation on joins。这值得一试。）

MySQL查看列为NULL

2 个答案: