![]() Code language: SQL (Structured Query Language) ( sql ) Creating a UNIQUE constraint on multiple columns ERROR: duplicate key value violates unique constraint "person_email_key"ĭETAIL: Key (email)=( ) already exists. VALUES( 'jack', 'doe', ' ') Code language: SQL (Structured Query Language) ( sql ) INSERT INTO person(first_name,last_name,email) Second, insert another row with duplicate email. VALUES( 'john', 'doe', ' ') Code language: SQL (Structured Query Language) ( sql ) Note that the UNIQUE constraint above can be rewritten as a table constraint as shown in the following query: CREATE TABLE person (įirst, insert a new row into the person table using INSERT statement: INSERT INTO person(first_name,last_name,email) ) Code language: SQL (Structured Query Language) ( sql ) The following statement creates a new table named person with a UNIQUE constraint for the email column. When you add a UNIQUE constraint to a column or a group of columns, PostgreSQL will automatically create a unique index on the column or the group of columns. It rejects the change and issues an error if the value already exists. The same process is carried out for updating existing data. When a UNIQUE constraint is in place, every time you insert a new row, it checks if the value is already in the table. PostgreSQL provides you with the UNIQUE constraint that maintains the uniqueness of the data correctly. ![]() Sometimes, you want to ensure that values stored in a column or a group of columns are unique across the whole table such as email addresses or usernames. ![]() You can define UNIQUE constraints at the table level and at the column level.Summary: in this tutorial, you will learn about PostgreSQL UNIQUE constraint to make sure that values stored in a column or a group of columns are unique across rows in a table. Similarly, all ADD REGION and DROP REGION statements will be blocked while an index is being modified on a REGIONAL BY ROW table within the same database.įor an example that uses unique indexes, see Add a unique index to a REGIONAL BY ROW table. While CockroachDB process an ADD REGION or DROP REGION statement on a particular database, creating or modifying an index will throw an error. Explicit index partitioning is not required. When you add an index to a REGIONAL BY ROW table, it is automatically partitioned on the crdb_region column. In multi-region deployments, most users should use REGIONAL BY ROW tables instead of explicit index partitioning. You can define the UNIQUE constraint when you create a table, or you can add it to an existing table through ADD CONSTRAINT. This does not mean that each value in each column must be unique, as if you had applied the UNIQUE constraint to each column individually. When using the UNIQUE constraint on multiple columns, the collective values of the columns must be unique. Conversely, dropping the UNIQUE constraint also drops the automatically created index. The UNIQUE constraint depends on the automatically created index, so dropping the index also drops the UNIQUE constraint. To avoid having two identical indexes, you should not create indexes that exactly match the UNIQUE constraint's columns and order. You can also achieve the same behavior through the table's Primary Key.Ĭolumns with the UNIQUE constraint automatically have an index created with the name _key. If you need to strictly enforce uniqueness, use the NOT NULL constraint in addition to the UNIQUE constraint. This means that it's possible to insert rows that appear to be duplicates if one of the values is NULL. ![]() You can insert NULL values into columns with the UNIQUE constraint because NULL is the absence of a value, so it is never equal to other NULL values and not considered a duplicate value. ![]() The UNIQUE constraint specifies that each non- NULL value in the constrained column must be unique. ![]()
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