Now, to add a unique constraints, click on the + button to add a row in a grid. FOREIGN KEY. Ensures that the value in a column or a set of columns matches the value in a column of another table. Here is an example of how to create a unique constraint on the email column of a users table: CREATE TABLE users ( id INTEGER SQL：在 PostgreSQL 中多个可为空列的唯一约束 在本文中，我们将介绍在 PostgreSQL 中如何定义多个可为空列的唯一约束。 阅读更多：SQL 教程 什么是唯一约束？ 唯一约束（Unique Constraint）是一种用于限制表中列值的重复性的约束。 The solution: an exclusion constraint instead of a unique constraint. Jul 30, 2010 · 28. pg_constraint. constraint_name. If this table is a partition, one cannot perform DROP NOT NULL on a column if it is marked NOT NULL in the parent table. ) Not-null constraints are represented in the pg_attribute catalog, not here. data is string only in both the column, Ya it should allow A not-null constraint is always written as a column constraint. Your index can now be: CREATE UNIQUE INDEX uix_custom_client_option ON test_table(custom, client, option) NULLS NOT DISTINCT; To note: null is treated as a value (that conflicts with another null) in all index columns of the same index this way, not just in option. In above statement, mytable is the name of the table you want to modify, column_name is the name of the column you want to make unique, and unique_column_name is a name you choose for the unique A not-null constraint is always written as a column constraint. 7. relname as table_name, c. PostgreSQL UNIQUE Constraint Syntax. create unique default data for the new column. Jan 29, 2015 · 19. 13. If the column type is boolean (likely should be), you can simplify: CREATE UNIQUE INDEX tbl_some_name_idx ON tbl (subset) WHERE type; This way you can even make combinations with NULL unique, which wasn't possible before Postgres 15. If you get this message when trying to insert data into a PostgreSQL database: ERROR: duplicate key violates unique constraint. By default, in PostgreSQL, two null values are not considered equal; however, PostgreSQL 15. col3 + 1; ERROR: there is no unique or exclusion constraint matching the ON CONFLICT specification. A table constraint is a rule that is applied to the data within the table to maintain data integrity. UNIQUE (email); Note that if there are existing duplicates in the column, PostgreSQL won’t let you add the constraint until those duplicates are resolved. It has been explained already that a constraint or unique index only cannot enforce the logic that you want. There can be multiple values of either value, so long as there are not two that share both. This post takes a look at an improvement to UNIQUE constraints on columns with NULL values. Is it better to create a separate raw query? Without a migration tool? If I use a raw query, can I use it like the address below? How to create composite UNIQUE constraint with nullable columns? Here is my code example. The SQL tab displays the SQL code generated by dialog selections. The catalog pg_constraint stores check, primary key, unique, foreign key, and exclusion constraints on tables. Dec 11, 2023 · In contrast, UNIQUE constraints in PostgreSQL allow for NULL values. What I've tried so far: UNIQUE(serviceproviderid, emailprefix, emaildomainid); Where serviceproviderid and emaildomainid are BIGINT, and emailprefix is TEXT. Apr 30, 2013 · If it is feasible to complete the entire operation within one transaction, it is possible to change the time which postgres evaluates the constraint, i. Be Mindful of Nullable Columns: If a column is nullable and has a unique constraint, multiple rows can contain NULL values, as NULL is considered distinct from every other value, including itself. If you want to forbid the null case, then it is not Oct 28, 2021 · Create unique constraint with null columns; Postgres 14 or older. Let’s explore some examples of using the PostgreSQL unique indexes. table_schema, tc. See: Create unique constraint with null columns; PostgreSQL multi-column unique constraint and NULL values To make a column unique in a PostgreSQL table, you can use the ALTER TABLE statement with the ADD CONSTRAINT clause. ); From what I understand both commands will generate an index on the two columns to enforce the unique constraint but with different ordering. May 28, 2017 · PostgreSQL behavior for unique constraints: " [T]wo null values are never considered equal in this comparison. id SERIAL PRIMARY KEY, uuid uuid NOT NULL UNIQUE, created timestamp with time zone NOT NULL, modified timestamp with time zone NOT NULL, deleted timestamp with time zone. ADD CONSTRAINT comments_can_only_be_saved_once_per_list. This syntax seems to be designed for a single composite unique constraint over two columns, rather than two constraints. You may be able to remove the unique CONSTRAINT, and not the INDEX itself. create or replace function table1_table2_cross_check() returns trigger language plpgsql as $$ begin if tg_table_name = 'table1' then if exists (select null from table2 where label = new. The above constraint works as intended when user_id and item_id are both specified: 1. 5 and trying to create an unique constraint based on 3 fields. Note that rows with NULL values in either a_id or b_id evade this restriction. The Unique constraint dialog organizes the development of a unique constraint through the following dialog tabs: General and Definition. create unique index on public. drop_all() methods do this by default, using a topological sort of all the Table objects involved such that tables are created and dropped in order of their foreign key dependency (this sort is also available via the A not-null constraint is always written as a column constraint. WHERE table_name = 'your_table'. This is the table that you wish to add a unique constraint to. So I would not need to generate a separate index to speed up queries like this in either case. constraint chk_education check (schoolName is not null or studiedAt is not null) From the manual: A check constraint is the most generic constraint type. Exclusion constraints are an extension of unique constraints: while a unique constraint prevents two rows with equal values for a column, exclusion constraints extend that from equality to other operators. Sometimes you want a foreign keyed column to be nullable because it is not required (just as not every citizen in a citizens table went to a university, so a university_id column can be null). To quote: If a unique constraint refers to a group of columns, the columns are listed separated by commas… This specifies that the combination of values in the indicated columns is unique across the whole table, though any one of the columns need not be I have problem with Postgres Unique constraint with multiple columns that may contain NULL value. This will depend on what database you're using. we should allow many archived records for a given (user_id, item_id) pairing). Ensures that all values in a column are unique across the dataset. NOT NULL Constraint − Ensures that a column cannot have NULL value. #. You may want to add a constraint without an index. I was able to list them using this below query from old answers to similar questions: SELECT. You should create two partial unique indexes. A not-null constraint is functionally equivalent to creating a check constraint CHECK (column_name IS NOT NULL), but in PostgreSQL creating an explicit not-null constraint is more efficient. Basic example, where username needs to be I want to create a unique constraint similar to this: ALTER TABLE Favorites. Oct 3, 2018 · UNIQUE(col2, col1) -- reversed column ordering. The MetaData. and table_schema = 'public'. Aug 30, 2023 · You can add a unique constraint to a table column using the ALTER TABLE statement. It guarantees that values within a column are not null. Ensures that each record in a table is unique and not NULL. See the section on Apr 3, 2021 · create table companies ( company_id serial primary key , company_name text not null , headquaters_city text , headquaters_address_line text , headquaters_zipcode text , constraint all_or_nothing_address check ((case when headquaters_city is not null then 1 else 0 end + case when headquaters_address_line is not null then 1 else 0 end + case when The syntax for creating a unique constraint using an ALTER TABLE statement in PostgreSQL is: ALTER TABLE table_name. Below is sample migration code for Postgres. select n. Jul 11, 2022 · By Ryan Lambert -- Published July 11, 2022. Unique constraints in PostgreSQL < 9. Partial Indexes #. ADD CONSTRAINT unique_email. Check your CONSTRAINTS via select * from information_schema. 以下是在 postgresql 中常用的约束。 not null：指示某列不能存储 null 值。 unique：确保某列的值都是唯一的。 primary key：not null 和 unique 的结合。确保某列（或两个列多个列的结合）有唯一标识，有助于更容易更快速地找到表中的一个特定的记录。 Jun 26, 2020 · LINE 1: ALTER TABLE shop ALTER COLUMN modified timestamp NULL. First, create a table called Nov 13, 2023 · Adding a unique constraint will automatically create a unique B-tree index on the column or group of columns listed in the constraint. To create a unique constraint in PostgreSQL, you can use the UNIQUE keyword followed by the name of column (s) you would like to id integer generated by default as identity primary key, t1_id int references t1(id) ); insert into t1 values (1); insert into t2 values (1, 1); -- OK, 1 exists so accepted. OR. So, col1 and col2 can repeat, but not at the same time. e. column_name, ccu. Mar 13, 2015 · Inn PostgreSQL I want to have a multi-column UNIQUE constraint where one of the columns can be NULL exactly once. 0 is currently not an option. ADD CONSTRAINT constraint_name UNIQUE (column1, column2, column_n); table_name. Dec 17, 2019 · A unique constraint is satisfied if and only if no two rows in a table have the same non-null values in the unique columns. Ive tried this but does not work. Postgres 15 beta 2 was released recently! I enjoy Beta season reviewing and testing new features is a fun diversion from daily tasks. Here's an example SQL statement. Edit: Alternative to comply with Pithyless Aug 13, 2021 · Specifying unique_id as PRIMARY KEY or UNIQUE in table definition; Running ALTER TABLE temp_table ADD PRIMARY KEY (unique_id) after creating temp_table; Regardless of what I do, I get the error: psycopg2. 22. Sep 13, 2016 · “Adding a unique constraint will automatically create a unique B-tree index on the column or group of columns listed in the constraint. I am using PostgreSQL and would like to prevent certain required CHARACTER VARYING (VARCHAR) fields from allowing empty string inputs. x introduced NULLS NOT DISTINCT (which we discuss later in this post). ADD CONSTRAINT Favorites_UniqueFavorite UNIQUE(UserId, MenuId, RecipeId); However, this will allow multiple rows with the same (UserId, RecipeId), if MenuId IS NULL. Pointing to the timestamp ^. PostgreSQL includes the following column constraints: NOT NULL – ensures that the values in a Sep 23, 2023 · Types of Constraints in PostgreSQL PRIMARY KEY. key_column_usage. Jul 22, 2014 · The solution is to remove your data, add the column then add the data back. That means that even in the presence of a unique constraint, it’s A not-null constraint is always written as a column constraint. create the column first, generate unique data for it, then add NOT NULL afterwards. Today, we are talking about two new features in Postgres 15. One way: (see demo). table_name, kcu. How to Add a Not Null Constraint in PostgreSQL. Oct 17, 2023 · This constraint should not apply to records with archived_at as a non null value (i. answered Dec 2, 2016 at 13:34. May 11, 2015 · Create unique constraint with null columns; The solution is very simple now: ALTER TABLE example ADD CONSTRAINT foo UNIQUE NULLS NOT DISTINCT (field1, field2, field3, field4, field5); For Postgres 14 or older. PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a table. Follows CREATE INDEX format. By default, NULL values are treated as distinct entries. I would like to set up a table in PostgreSQL such that two columns together must be unique. UNIQUE. the table itself looks. 0 aren't deferrable and unfortunately using 9. One exception is when a CharField has both unique=True and blank=True set. Partial indexes are a specialized feature, but there are several situations in which Jun 1, 2023 · PostgreSQL supports unique constraints to ensure that the values in a particular column ( or set of columns ) are unique across all rows in a table. In this situation, null=True is required to avoid unique constraint violations when saving multiple objects with blank values. Oct 18, 2022 · 2. The primary key constraint specifies that a column or columns of a table can contain only unique (non-duplicate), nonnull values. g. The name of the table to modify. Below is the syntax add a unique constraint: where table_name is the name of the table where you want to add the unique constraint. These fields would also need to contain unique values, so I am already using a unique constraint; however, this does not prevent an original (unique) empty value. Thanks 53. The following are commonly used constraints available in PostgreSQL. Overall though, I’m impressed with the functionality that is possible with the constraints in PostgreSQL. Apr 20, 2009 · AS (Coalesce(SamAccountName, Convert(varchar(11), PartyID))) ALTER TABLE dbo. Finally Dec 9, 2022 · I have a database with a unique constraint in a field that is apparently being ignored. NULL “ Constraint ” The NULL “ constraint ” (actually a non-constraint) is a PostgreSQL extension to the SQL standard that is included for compatibility with some other database A not-null constraint is always written as a column constraint. Party ADD CONSTRAINT UQ_Party_SamAccountName. The problem I'm having is two of the columns can be nullable so rows with these fields as NULL are not seen to breach the unique constraint. ALTER TABLE favorite_comments. There can already be multiple rows with (col2, col3) = (1, NULL). The logic is to use window functions to create an additional column in the view that resets every two rows having the same (user_id, enabled). This enforces a field to always contain a value, which means that you cannot insert a new record, or update a record without adding a value to this field. I could just add unique index. for reference another answer with more info: Create unique constraint with null columns. Since Postgres doesn't consider null values to invoke unique constraint, whenever any of nullable columns have null value, a new row is inserted, instead of update. Sure, you could perform this validation in your application layer, but shit happens: somebody will forget to add the validation, somebody will remove it by accident, somebody will bypass validations in a console and insert nulls, etc. 3 Doc Apr 27, 2016 · INSERT INTO mytable(col1, col2, col3) VALUES ('A', 'B', 0) ON CONFLICT (col1, col2) DO UPDATE SET col3 = EXCLUDED. If you want to know more about NULL in general, check out my post about NULL values in PostgreSQL. 1) Unique indexes for a primary key column and a column with a unique constraint. alter table file drop constraint file_file_name_key; If you don't want to rely on the default naming strategy, you can use the following query to retrieve the name: select constraint_name. It is a PostgreSQL extension that a foreign key constraint may reference columns of a unique index instead of columns of a primary key or unique constraint. Again click on the Nov 5, 2019 · I would like to add a constraint that checks a conditional uniqueness, UNIQUE(opinion_id, list_id) OR UNIQUE(review_id, list_id), but since opinion_id, review_id can be NULL im having some trouble with this. That table also includes two rows with the same value for that field, but somehow querying by that value only returns one row. Jul 14, 2022 · July 14, 2022. 4 Doc - Set constraints or Postgres 8. errors. UNIQUE Constraint − Ensures that all values in a column are different. Sep 29, 2023 · The UNIQUE constraint is similar to the primary key except that the UNIQUE constraint accepts the null values. That means, your current table definition does not do what you say it should do. Specifying NULLS NOT DISTINCT on unique indexes / constraints will cause NULL to be treated as not distinct, or in other words, equivalently. FROM information_schema. label ) then raise exception 'Executing: % table1, Label Value: ''%'', already exists in table2',tg_op Jan 23, 2024 · PostgreSQL allows null values in columns defined as unique or the columns used in a multicolumn unique constraint. create_all() and MetaData. In PostgreSQL, a CHECK constraint ensures that values in a column or a group of columns meet a specific condition. Open Table Properties. The column definitions of the specified columns do not have to include a NOT NULL constraint to be included in a UNIQUE constraint. See: Create unique constraint with null columns Aug 14, 2021 · When values for all columns are present this setup works perfectly but, it's not the case when any of nullable columns have a null value. (tracking_number, carrier) is a list of the columns you want to use for the constraint. Constraints. PostgreSQL rejects the submission since it does not pass the final table check constraint. . That means even in the presence of a unique constraint it is possible to store duplicate rows that contain a null value in at least one of the constrained columns. 8. How to add not null constraints in PostgreSQL A not-null constraint is always written as a column constraint. I'm aiming for this to be a constraint as I am trying to do an update on conflict (UPSERT). When you define a primary key or a unique constraint for a table, PostgreSQL automatically creates a corresponding unique index. FOREIGN Key − Constrains data based on columns in other tables. First of all, we're going to talk about the new unique constraint NULLS NOT DISTINCT option, and then we'll talk about the SELECT DISTINCT performance improvements. FOREIGN KEY directive is used to create the constraint in an “inline” fashion within the CREATE TABLE definition. Any non-null unique set of columns can be chosen to be a primary key for a table; thus, there is no programmatic way to determine the best Feb 2, 2023 · A few things like the ability to control how NULL values are resolved in a unique constraint, or the very concept of exclusion constraints will be new. constraint_name, tc. In the popup, go to the 'Constraints' tab and click on the Unique tab, as shown below. Moreover, you can have as many unique constraints as you want on a single table. While the nuances of unique constraints are not as flashy as making sorts I have a feeling you might be confusing single column constraints with multiple column constraints, or your data is not what you expect it to be. See: Postgres 7. The NOT NULL constraint enforces a column to NOT accept NULL values. A unique index (or constraint) allows multiple rows with NULL in them. class AddUniqeConstraintToShipments < ActiveRecord::Migration. You can create the unique constraint in PostgreSQL in multiple ways. Therefore, the two referencing_table For more info, see the Postgres doc for Constraints, the "Unique Constraints" section. PRIMARY Key − Uniquely identifies each row/record in a database table. A partial index is an index built over a subset of a table; the subset is defined by a conditional expression (called the predicate of the partial index). Add Unique Constraint. It's perfectly acceptable to reference a unique column containing NULL entries, in other words: We can nicely reference primary keys as well as unique constraints equally - there are absolutely no differences to worry about. The NOT NULL constraint is much more focused. ” PostgreSQL Documentation — DDL Constraints . Let's assume this situation: Insert will insert ('foo', 'bar') once and ('foo', NULL) twice (even though the intuition says it should insert once). Unique constraint can be defined on one or more columns of a table. Jan 1, 2023 · To create a unique constraint in PostgreSQL, you can use the CREATE TABLE or ALTER TABLE statement, depending on whether you want to add the constraint to an existing table or create a new table with the constraint. 117k 15 214 241. CHECK Oct 9, 2020 · CREATE UNIQUE INDEX tbl_some_name_idx ON tbl (subset) WHERE type = 'true'; Data type does matter. You are striving for compatibility with your existing Oracle and SQL Server implementations. Bold emphasis mine. (Column constraints are not treated specially. Any indexes that satisfy the predicate (which need not actually be partial indexes) can be inferred. May 19, 2015 · Adding UNIQUE constraints to tables in Postgres is very easy! Imagine we have the following table: CREATE TABLE users ( id uuid PRIMARY KEY NOT NULL DEFAULT uuid_generate_v4(), email text ); If we want to ensure that each user has a unique email we simply add: ALTER TABLE users ADD CONSTRAINT email_unique UNIQUE (email); Jun 18, 2024 · The CONSTRAINT. The index contains entries only for those table rows that satisfy the predicate. The UNIQUE constraint does exactly what you want. NULL values are not considered equal to each other and thus never trigger a UNIQUE violation. This is a misunderstanding. 11. (This deviates from the SQL92 definition, but is a more sensible convention. (emphasis mine) (emphasis mine) I'm having trouble finding a clearer definition of exactly how to treat null values, but this stack overflow answer interprets it and subsequent SQL standards to mean that duplicate nulls Unique constraints ensure that the data contained in a column, or a group of columns, is unique among all the rows in the table. The only way you can prevent that is to either declare the column as NOT NULL in order to force a value for it, or reduce the unique columns to (user_id, class_type_id) answered May 3, 2014 at 20:10. Use ON CONFLICT Clause : When inserting data into a table with unique constraints, the ON CONFLICT clause can be used to define how PostgreSQL should Introduction to PostgreSQL CHECK constraints. tablename (short_code_3, deleted) where deleted is not null; create unique index on public. Alternatively you can use pg_constraint to retrieve this information. UNIQUE (SamAccountNameUnique) You simply have to put something in the computed column that will be guaranteed unique across the whole table when the actual desired unique column is NULL. klin. If you have a sequencer on the field (like with a serial column type), it is likely out of sync with the table because you have done an insert and supplied a value (rather than letting the "default" mechanism handle it by getting the nextval from the sequencer). By default, a column can hold NULL values. May 3, 2014 · This happens because of the NULL value in the created_at column. Jun 2, 2017 · Because PostgreSQL can not infer it from the values, you need the index_predicate. Another PostgreSQL extension to standard SQL are exclusion constraints. from information_schema. For instance: id int PRIMARY KEY AUTOINCREMENT, col1 int NOT NULL, col2 int NOT NULL. An alternative approach would be to use a materialized view. If those columns are numeric or float (rather than integer or bigint) using '-Infinity' might be a better substitution value. Aug 25, 2019 · Since Postgres 15, there is a way with the added clause NULLS NOT DISTINCT. I ran a test to confirm that NOT NULL is completely redundant in combination with a PRIMARY KEY constraint (in the current 83. Jan 21, 2023 · You cannot create a unique constraint across table, but a trigger on both tables can. The index covers the columns that make up the primary key or unique constraint (a multicolumn index, if appropriate), and is the The column definitions of the specified columns do not have to include a NOT NULL constraint to be included in a UNIQUE constraint. index_predicate Used to allow inference of partial unique indexes. : START; SET CONSTRAINTS <> DEFERRED; <SOME INSERT/UPDATE/DELETE> COMMIT; In this case, the constraint is evaluated at commit. 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. Mar 6, 2019 · So you can use. constraint_table_usage. While this is a simple constraint, it is used very frequently. A uniqueness restriction covering only some rows cannot be written as a unique constraint, but it is possible to enforce such a restriction by creating a unique partial index. I want to allow NULL in MenuId to store a favorite that has no associated menu, but I only want at most Dec 2, 2016 · 2. You can use a check constraint e. Or use the NULLS NOT DISTINCT clause in Postgres 15 or later. PostgreSQL UNIQUE index examples. CREATE TABLE shop (. Jun 25, 2014 · Obviously, from the message, you are trying to insert a value in a column that already exists there. Note that some column constraints can be defined as table constraints such as primary key, foreign key, unique, and check constraints. InvalidColumnReference: there is no unique or exclusion constraint matching the ON CONFLICT specification. conname as constraint_name. In other cases, the column should not be null, just as every student lshould be associated with a university_id. NULL “ Constraint ” The NULL “ constraint ” (actually a non-constraint) is a PostgreSQL extension to the SQL standard that is included for compatibility with some other database Not null. I'm running Postgres 9. The manual: In general, a unique constraint is violated when there is more than one row in the table where the values of all of the columns included in the constraint are equal. May 12, 2022 · CREATE UNIQUE INDEX a_special_uni_idx ON a (a_id, b_id) WHERE some_value IS NULL; This allows each combination of (a_id, b_id) only once when some_value is null. See: Now, right-click on the table where you want to add the unique constraints and click on 'Properties' in the menu. Ordinarily this is checked during the ALTER TABLE by scanning the entire table; however, if a valid CHECK constraint is found which proves no NULL can exist, then the table scan is skipped. Not null constraints are a great way to add another layer of validation to your data. Every column constraint is equivalent to some table constraint. table_constraints; Then if you find one, you should be able to drop it like: ALTER TABLE <my_table> DROP CONSTRAINT <constraint_name>. A check constraint uses a boolean expression to evaluate the values, ensuring that only valid data Mar 4, 2012 · 36. table_schema AS foreign_table_schema, Nov 2, 2021 · How to create unique constraint with nullable field in TypeORM? I used postgresql12, node v12. Having more than one null value in a column without a NOT NULL constraint, does not violate a UNIQUE constraint. Technically, PRIMARY KEY is merely a combination of UNIQUE and NOT NULL. A not-null constraint is always written as a column constraint. Also speeds up queries with conditions on one or both column. PostgreSQL treats null as a distinct values, so you will be able to have multiple null values in a column with the unique index constraint. Not null constraints. A unique constraint ensures that all values in a column are unique, meaning no two rows can have the same value in that column. nspname as schema_name, t. In this scenario solution is pretty straightforward. That likely means that the primary key sequence in the table you're working with has somehow become out of sync, likely because of a mass import process (or something along those lines). The drawback is that you cannot give explicit names to not-null constraints created this way. insert into t2 values (2, null); -- OK, null is accepted. SELECT id, col3 FROM foo WHERE col1 = 'stack' AND col2 This way you can enter (1,2), (1,3) and (1, null), but neither of them a second time. I'd like to figure a better way to list all UNIQUE constraints in a specific schema. tc. There is a drawback to this though: Jan 17, 2022 · 184. You need to retrieve the table names by running the following query: SELECT *. where table_name = 'file'. emaildomainid is the only column with NULL allowed, and is the column I'm May 22, 2020 · 1. and column_name = 'file_name'; Dec 6, 2019 · create unique index on the_table (coalesce(a,-1), coalesce(b, -1), coalesce(c, -1)); That way NULL values are treated the same inside the index, without the need to use them in the table. It allows you to specify that the value in a certain column must satisfy a Boolean (truth-value) expression. tablename (short_code_3) where deleted is null; (I don't know how to do it in your ORM). 5mins of Postgres E26: New in Postgres 15 - NULL handling in UNIQUE constraints & Parallel DISTINCT. A check constraint allows you to enforce data integrity rules at the database level. Sep 28, 2021 · In most cases, it’s redundant to have two possible values for “no data;” the Django convention is to use the empty string, not NULL. insert into t2 values (3, 2); -- NOK, 2 is not accepted. Feature Description UNIQUE NULLS NOT DISTINCT. Jan 4, 2024 · The following command shows how to add a unique constraint to the email column of the users table: ALTER TABLE users. This results in a unique constraint violation: ERROR: duplicate key value violates unique constraint The problem seems to be the order in which PostgreSQL performs the updates. See the section on A multicolumn unique index will only reject cases where all indexed columns are equal in multiple rows. answered Jan 10, 2023 at 19:53. NULL values can coexist in multiple rows in a column defined UNIQUE. A table can only have a single primary key (though a primary key is not required), but can also have multiple unique constraints. up pc zd vf tr ms tw ac gj zb