alter table: only allow either unique or empty
132527
Unique constraint that allows empty values in MySQL
113435
Dropping Unique constraint from MySQL table
155730
Create a function with optional arguments in MySQL
32901
Adding a constraint to prevent duplicates in SQL Update Trigger
121322
How do you drop a default value or similar constraint in T-SQL?
20342
Checking the way to stop duplicate names - validation rules or triggers
8161
How do I create a stored procedure that will optionally search columns?
76542
Enforcing unique values in field when editing in ArcMap?
154268
How to implement a 'default' flag that can only be set on a single row
67415
sql unique constraint on a 2 columns combination
157207
Remove trivial edit restriction
67433
Insert data only if record does not exist
114718
Is there a DBMS that allows the "= NULL" syntax?
112077
How to check if a value already exists to avoid duplicates?
54371
Specific merge 2 columns in sql excluding one of them
84131
Display error message if same record exist with combination of some fields?
3556
Select all columns except one in MySQL?