SQL Query to get records from a child table which does not have a parent in the parent table?

Question

FK列可能为null或其他值（不是父表中的PK）。

Answer 1

你可以试试这个

Select * from child_table where FK not in (select PK from parent_table)

Answer 2

Try this

    SELECT ChildTable.ParentID
    FROM ChildTable
    WHERE NOT EXISTS (
    SELECT 1 FROM ParentTable
    WHERE ChildTable.ParentID = ParentTable.ParentID
    )

Answer 3

You can use LEFT JOIN to get the expected result.

Please refer the following working example with sample data:

DECLARE LOCAL TEMPORARY TABLE ParentTable TABLE (Id INT IDENTITY(1,1) , [Name] VARCHAR (100));

DECLARE LOCAL TEMPORARY TABLE ChildTable TABLE (Id INT IDENTITY(1,1), ParentId INT NULL, [Value] VARCHAR (50));

INSERT INTO ParentTable VALUES ('Name 001'), ('Name 002'), ('Name 003');

INSERT INTO ChildTable VALUES (1, 'Val 01'), (2, 'Val 02'), (NULL, 'Val 03'), (4, 'Val 04');

SELECT C.*
FROM ChildTable C
LEFT JOIN ParentTable P ON P.Id = C.ParentId
WHERE P.Id IS NULL;