在sqlserver可空列中创建唯一约束，但是允许多个null值
https://blog.csdn.net/mynameisfish/article/details/84562713
CREATE UNIQUE NONCLUSTERED INDEX idx_yourcolumn_notnull
ON YourTable(yourcolumn)
WHERE yourcolumn IS NOT NULL;
https://blog.csdn.net/mynameisfish/article/details/84562713
CREATE UNIQUE NONCLUSTERED INDEX idx_yourcolumn_notnull
ON YourTable(yourcolumn)
WHERE yourcolumn IS NOT NULL;