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How do I use ALTER TABLE to add a new column and make it unique?
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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);
In Object Explorer, right-click the table to which you want to add columns and choose Design. Select the first blank cell in the Column Name column. Type the column name in the cell. The column name is a required value.
The basic syntax of an ALTER TABLE command to add a New Column in an existing table is as follows. ALTER TABLE table_name ADD column_name datatype; The basic syntax of an ALTER TABLE command to DROP COLUMN in an existing table is as follows.
The syntax for creating a unique constraint using an ALTER TABLE statement in MySQL is: ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column1, column2, ... column_n); table_name.
Depends on the DBMS, but I think the following is quite portable:
ALTER TABLE table_name ADD column_name datatype
ALTER TABLE table_name ADD UNIQUE (column_name)
If you want to give a name to the UNIQUE constraint, you could replace the last command with this:
ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column_name)
if table is empty
ALTER TABLE ADD (FieldName Type)
ALTER TABLE ADD CONSTRAINT UNIQUE(FieldName)
If you have data in table you need to this in three steps:
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