Question

我需要创建一个完整性约束，以确保在一列表中只有一个空值。经典的UNIQUE不好用，因为它没有检测到多个空值...... 我怎么办？

Answer 1

CREATE TABLE qwe(
  id int
);

CREATE UNIQUE INDEX qwe_idx ON qwe( 
      CASE WHEN id IS null THEN 'NULL' ELSE to_char(id) END 
);

INSERT INTO qwe VALUES(1);
1 row inserted.

INSERT INTO qwe VALUES(1);
Error starting at line : 9 in command -
INSERT INTO qwe VALUES(1)
Error report -
ORA-00001: naruszono więzy unikatowe (TEST.QWE_IDX)

INSERT INTO qwe VALUES(NULL);
1 row inserted.

INSERT INTO qwe VALUES(NULL);
Error starting at line : 9 in command -
INSERT INTO qwe VALUES(NULL)
Error report -
ORA-00001: naruszono więzy unikatowe (TEST.QWE_IDX)

SELECT * FROM qwe;

        ID
----------
         1
    (null)

添加具有空值的约束唯一列

1 个答案: