Partial value constraint of sql server table

问题

I have a SQL Server table with two columns id:INT and flagged:BOOLEAN. Is it possible to add a constraint that ensures that there is only one entry for (id=a, flagged=b) where b = 1?

For example:

ok (id=1, flagged=1) (id=1, flagged=0) (id=1, flagged=0) not ok (id=1, flagged=1) (id=1, flagged=1) (id=1, flagged=0)

回答1:

Create Unique Index with filter:

CREATE UNIQUE INDEX idx_name ON your_table(id)
   WHERE flagged=1;

Demo:

SqlFiddleDemo

CREATE TABLE your_table(id INT, flagged INT);

CREATE UNIQUE INDEX idx_name ON your_table(id)
   WHERE flagged=1;


INSERT INTO your_table(id, flagged)
VALUES (1, 0), (1,1), (1,0);

INSERT INTO your_table(id, flagged)   -- will fail
VALUES (1,1);

/* Cannot insert duplicate key row in object 'dbo.your_table'
   with unique index 'idx_name'. The duplicate key value is (1).*/

来源：https://stackoverflow.com/questions/32473877/partial-value-constraint-of-sql-server-table