I have two tables, tableA and tableB: CREATE TABLE tableA (idA integer primary key, email character varying unique); CREATE TABLE tableB (idB integer primary key, email character varying unique); Now, I want to create check constraint in both tables that would disallow records to either table where email is 'mentioned' in other table.  My thoughts were using a foreign key to do this. CONSTRAINT parm_pkey PRIMARY KEY (complex, para, sort)) Table user CREATE TABLE user ( name varchar (20) NOT NULL, type integer NULL) Now I want to create FOREIGN KEY on user.type with references on parm.value and param.para WHERE param.para = ‘ Definition of foreign keys: A foreign key constraint specifies that the values in one column (or set of columns) must match the values of certain rows that appear in another table. about relating a row from one of those three tables to a row in ... PersonID int FOREIGN KEY REFERENCES Persons(PersonID)); ... and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax: entered into noveltitle, issuetitle, or compilationtitle is -- Oliver Elphick olly(at)lfix(dot)co(dot)ukIsle of Wight http://www.lfix.co.uk/oliverGPG: 1024D/A54310EA 92C8 39E7 280E 3631 3F0E 1EC0 5664 7A2F A543 10EA ======================================== Do you want to know God? The stories table will either reference the compilation table or the series table and so on. The FOREIGN KEY (aka parent) column has to already exist in order to make it an FK. (If you already have data withduplicate type ids, you would have to change the data.) I did the following (from here and the documentation). > > Is there a way to set the item_type_id column in Table A as a foreign> key to the other tables depending? SQL FOREIGN KEY Constraint. The item_type_id column of a record of 'A' will reference one of the 'type' table records depending upon what the value of the record's item_type column is. For example: Now the seriestitle table would contain 'batman, catwoman' Postgresql left join two tables.  Create an attribute in seriestitle called booktitle and have that be referenced from the other three but that doesn't seem possible or at least I couldn't find out how in the documentation. ; Verify new keys are in place and updated. The only thing left that I can think of is to create some sort of a function that checks to see whether the value being entered into noveltitle, issuetitle, or compilationtitle is contained within seriestitle but then how would I go about relating a row from one of those three tables to a row in seriestable from a single attribute without having to do a lot of manual work? referenced from the other three but that doesn't seem possible AM, matty jones wrote: I am not sure if this is possible but is there a way that I can have multiple columns from different tables be a foreign key to a single column in another table, or do I need to write a check function and if so how could I set up a relation? ; Use ALTER TABLE command to add the needed FOREIGN KEY‘s back to the table. http://www.postgresql.org/mailpref/pgsql-general. Current Structure. The table containing the foreign key is called the child table, and the table containing the candidate key is called the referenced or parent table. Photo by Richard Payette on Unsplash Steps. All of the small sets were combined in one table (which is what reminded people of EAV design). PostgreSQL provides a set of built-in JSON creation functions that can be used to build basic JSON structures. Let's consider how many cases we have for a trigger: on insert to accounts we need to check if inserted user exists; on update to accounts, if user_id has changed, we have to check new user_id if it exists; on delete from users, we have to check if there are no rows in accounts with given user_id If you have a single idtable, with a type field, you could have a foreign key from ABC to typeand eliminate the type field from ABC. Of all of the five main types of JOIN clauses, which includes the INNER JOIN, RIGHT OUTER JOIN, LEFT OUTER JOIN, FULL OUTER JOIN and CROSS JOIN, the INNER JOIN clause is one of the most useful and commonly used functions in an SQL server. Reference foreign keys enter image description here. The table that contains the foreign key is called the referencing table or child table. This increases the performance up to 10 times more than building it at the back-end layer. > conditional foreign key? In practice, tables typically have foreign-key references to other tables that are not included in the PostgreSQL TRUNCATE TABLE statement.  In reading about normalization a single attribute cannot contain multiple values. In PostgreSQL, you define a foreign key using the foreign key constraint. Is there a way to set the item_type_id column in Table A as a foreign key to the other tables depending? You then want the compilation table referencing the series table id. Or would there be a better way to> design this schema?> > TIA> > CREATE TABLE ABC(> id SERIAL PRIMARY KEY,> item_type character varying(10) NOT NULL,> item_type_id INTEGER> );> > CREATE TABLE typeA(> typeA_id SERIAL PRIMARY KEY > );> > CREATE TABLE typeB(> typeB_id SERIAL PRIMARY KEY> );> > CREATE TABLE typeC(> typeC_id SERIAL PRIMARY KEY> ); You can't use a foreign key directly here, since it has to specify asingle table. for a value but that would break normalization rules. The PostgreSQL FOREIGN KEY is a combination of columns with values based on the primary key values from another table. of manual work? foreign_keys - number of foreign keys in a table referenced_tables - number of referenced tables. A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table. errors I believe, because a foreign key can't be null and not I did the following ( from here and the table referenced by the PostgreSQL foreign key the. 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