Question

我有以下SQL代码，它返回数据库中的表，以及每个表中的主键字段。

SELECT Keys.TABLE_NAME As 'Table Name',
Keys.COLUMN_NAME AS 'Primary Key'
FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS AS Constraints
JOIN INFORMATION_SCHEMA.KEY_COLUMN_USAGE AS Keys
ON Constraints.TABLE_NAME = Keys.TABLE_NAME
AND Constraints.CONSTRAINT_NAME = Keys.CONSTRAINT_NAME
WHERE Constraints.CONSTRAINT_TYPE = 'PRIMARY KEY'

实际上，它只显示那些具有主键的表。如何修改sql以显示所有表，而那些没有主键的表将在“Primary Key”列中显示“null”？

Answer 1

为此，您需要从所有表的列表开始，然后使用左外连接来连接表：

SELECT t.TABLE_NAME As 'Table Name',
       Keys.COLUMN_NAME AS 'Primary Key'
FROM INFORMATION_SCHEMA.TABLES t left outer join
     INFORMATION_SCHEMA.TABLE_CONSTRAINTS Constraints
     on t.TABLE_NAME = Constraints.Table_name and
        t.Table_Schema = Constraints.Table_Schema left outer join
     INFORMATION_SCHEMA.KEY_COLUMN_USAGE AS Keys 
     ON Constraints.TABLE_NAME = Keys.TABLE_NAME and
        Constraints.CONSTRAINT_NAME = Keys.CONSTRAINT_NAME and
        Constraints.CONSTRAINT_TYPE = 'PRIMARY KEY'

Answer 2

默认情况下，“join”是内部联接，如果根据联接条件在另一个表中存在相应的行，则只会显示一行。因此，您不希望在此处使用直接连接，您需要左连接，右连接或完全连接，这允许表返回没有匹配的行。

但是在这里你不能只是切换到左连接，因为table_constraints只列出有约束的那些，并且你的whereint约束将过滤掉没有主键的行。

因此，您可以引入information_schema.tables以包含每个表，然后将主键条件移动到连接条件而不是where子句。它可能看起来像：

SELECT t.TABLE_NAME As 'Table Name',  
Keys.COLUMN_NAME AS 'Primary Key'  
FROM 
INFORMATION_SCHEMA.TABLES as t
left join 
INFORMATION_SCHEMA.TABLE_CONSTRAINTS AS Constraints  
on t.TABLE_NAME = constraints.TABLE_NAME and t.TABLE_SCHEMA = constraints.TABLE_SCHEMA
left JOIN INFORMATION_SCHEMA.KEY_COLUMN_USAGE AS Keys  
ON Constraints.TABLE_NAME = Keys.TABLE_NAME  
AND Constraints.CONSTRAINT_NAME = Keys.CONSTRAINT_NAME  
and Constraints.CONSTRAINT_TYPE = 'PRIMARY KEY'

如果字段不满足条件，如何显示null

2 个答案: