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The syntax for creating a unique constraint using an ALTER TABLE statement in SQL Server is: ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column1, column2, ... column_n); table_name.
The constraint can be created within the CREATE TABLE T-SQL command while creating the table or added using ALTER TABLE T-SQL command after creating the table. Adding the constraint after creating the table, the existing data will be checked for the constraint rule before creating that constraint.
The syntax for creating a unique constraint using an ALTER TABLE statement in PostgreSQL is: ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column1, column2, ... column_n); table_name.
psql's inline help:
\h ALTER TABLE
Also documented in the postgres docs (an excellent resource, plus easy to read, too).
ALTER TABLE tablename ADD CONSTRAINT constraintname UNIQUE (columns);
Yes, you can. But if you have non-unique entries on your table, it will fail. Here is the how to add unique constraint on your table. If you're using PostgreSQL 9.x you can follow below instruction.
CREATE UNIQUE INDEX constraint_name ON table_name (columns);
If you had a table that already had a existing constraints based on lets say: name and lastname and you wanted to add one more unique constraint, you had to drop the entire constrain by:
ALTER TABLE your_table DROP CONSTRAINT constraint_name;
Make sure tha the new constraint you wanted to add is unique/ not null ( if its Microsoft Sql, it can contain only one null value) across all data on that table, and then you could re-create it.
ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column1, column2, ... column_n);
Yes, you can add a UNIQUE constraint after the fact. However, if you have non-unique entries in your table Postgres will complain about it until you correct them.
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