Let’s create a new table
books, and insert a few rows of records in it.
Notice that we are allowed to insert a book with exact same name (
Basics of SQL) twice, in the above set of statements. Sometimes, this may not be something that’s acceptable in a real world database scenario.
To ensure uniqueness in records under a column, SQL provides another
UNIQUE. Look at the statement below
UNIQUE next to the column name and data type, in the list of columns in
CREATE TABLE statement. This makes sure that no record is allowed which has the exact same value in the same column as an existing record. Try the set of statements below:
Notice the error that shows up. It expresses that the
UNIQUE constraint failed. Look at the table
books in the newly created tab. You’ll see that the first two
INSERT statements are successful, hence the two records in
books. It is the last statement, that violates the
UNIQUE constraint on
name, and throws an error.