To create a UNIQUE constraint on the "ID" column when the table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Persons
ADD UNIQUE (ID);
ADD UNIQUE (ID);
To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Persons
ADD CONSTRAINT UC_Person UNIQUE (ID,LastName);
ADD CONSTRAINT UC_Person UNIQUE (ID,LastName);
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