Showing Foreign key in the ER diagram is Valid or not?
I am showing the foreign key in erd, is this correct?
ER is a conceptual model and it have entity types connected by relationes, it never shows a foreign key.
Related
In the PostgreSQL 11 Release Notes I found the following improvements to partitioning functionality:
Add support for PRIMARY KEY, FOREIGN KEY, indexes, and triggers on partitioned tables
I need this feature and tested it.
Create table:
CREATE TABLE public.tbl_test
(
uuid character varying(32) NOT null,
registration_date timestamp without time zone NOT NULL
)
PARTITION BY RANGE (registration_date);
Try to create Primary key:
ALTER TABLE public.tbl_test ADD CONSTRAINT pk_test PRIMARY KEY (uuid);
I get an error SQL Error [0A000]. If use composite PK (uuid, registration_date) then it's work. Because PK contains partitioning column
Conclusion: create PK in partitioning tables work with restrictions (PK need contains partitioning column).
Try to create Foreign key
CREATE TABLE public.tbl_test2
(
uuid character varying(32) NOT null,
test_uuid character varying(32) NOT null
);
ALTER TABLE tbl_test2
ADD CONSTRAINT fk_test FOREIGN KEY (test_uuid)
REFERENCES tbl_test (uuid);
I get an error SQL Error [42809]. It means FOREIGN KEY on partitioning tables not work.
Maybe i'm doing something wrong. Maybe somebody tried this functionality and know how this work.
Maybe somebody know workaround except implement constraint in the application.
PostgreSQL v12.0 will probably support foreign keys that reference partitioned tables. But this is still not guaranteed as v12.0 is still in development.
For v11 and lower versions, you may use triggers as described by depesz in these posts: part1, part2, and part3.
Update: PostgreSQL v12.0 was released on Oct 3, 2019, with this feature included
Postgres 11 only supports foreign keys from a partitioned table to a (non-partitioned) table.
Previously not even that was possible, and that's what the release notes are about.
This limitation is documented in the chapter about partitioning in the manual
While primary keys are supported on partitioned tables, foreign keys referencing partitioned tables are not supported. (Foreign key references from a partitioned table to some other table are supported.
(emphasis mine)
I have table that has a TEXT primary key
CREATE TABLE tbl1{
a1 TEXT PRIMARY KEY,
...
);
(the a1 column is a foreign key inside another table)
How can I change values of a1?
If I do
UPDATE tbl1 SET a1 = ? WHERE a1 = ?
I get a constrain violation error
You should never change primary keys; it would be a better idea to use an INTEGER PRIMARY KEY and have the actual URL be a normal data column.
If you really want change a key that is the target of a foreign key, you should declare the foreign key constraint as deferred so that you are able to adjust the foreign key value in the same transaction.
The problem is that your table has single column that is the primary key and is a foreign key to another table. This suggests that the database design of the database is wrong.
Unless you can change the database structure you need to add the correct values in that other table to change your primary key value.
That is "insert into table constraintingTable(key,val) values (A,B)" and then execute update tbl set a1 = A where a1 = KEY.
Ignore the people telling you that primary keys should never be changed, there is a body of theory on how primary keys should be built. A primary key should uniquely identify the value columns of a row (see database theory), for instance typical keys are PNR, SSN, Serial Number, Mobile Phone number, and sometimes multi values like Name, Address, Street, Country. Generated keys should only be used if you generate new values or you have practical problems using a proper primary key.
here,i have seen diagram in which there is no primary key for entity.
in this diagram,item's order_level and price has no primary key.if there is not primary key then it is how to show in data dictionary?
It's one of:
1.) All attributes combined are it's primary key
2.) It's a weak identity and the diagram you are showing is incomplete
3.) The diagram is wrong/not showing the primary key. (Seen this many times in textbooks where ER diagrams are explained by example and the key is often missing if it's not needed to explain whatever.)
I am trying to change the type in two tables of an innoDB. The problem is that the values are a key and a foreign key. When I try to make the change I get the following error
#1025 error on rename
Do I need to drop the foreign keys and then make the changes and then reapply the foreign key?
Since u can use the name to drop the foreign key first and the column then:
ALTER TABLE categories DROP FOREIGN KEY categories_ibfk_1;
ALTER TABLE categories DROP COLUMN assets_id;
To find out which table caused the error you can run
SHOW ENGINE INNODB STATUS\G
and then look at the "LATEST FOREIGN KEY ERROR" section.
Yeah, you have to drop the foreign key. Try SHOW INNODB STATUS to see if there's a more elaborated explanation of what's going on.
I know that SQLite does not enforce foreign keys natively, but that's not my primary concern. The question is: If I declare
CREATE TABLE invoice (
invoiceID INTEGER PRIMARY KEY,
clientID INTEGER REFERENCES client(clientID),
...
)
will sqlite at least use the information that clientID is a foreign key to optimize queries and automatically index invoice.clientID, or is this constraint a real no-op?
In the SQLite Documentation it says:
... "an index should be created on the child key columns of each foreign key constraint"
ie. the index is not automatically created, but you should create one in every instance.
Even if it is not actually a no-op (a data structure describing the constraint is added to the table), foreign key related statement doesn't create any index on involved columns.
Indexes are implicitly created only in the case of PRIMARY KEY and UNIQUE statements.
For more details, check it out build.c module on the sqlite source tree:
http://www.sqlite.org/cvstrac/rlog?f=sqlite/src/build.c https://www.sqlite.org/src/file?name=src/build.c&ci=tip