I recently created a relational database model and it has a lot of primary key and foreign key relations. I want to use clickhouse for my database but it turns out that clickhouse does not support foreign key and unique primary keys. Can someone tell me if I am missing anything here.
You are right. CH does not have unique & foreign constraints.
Moreover JOINs are not the best part of ClickHouse.
ClickHouse suggests to create single wide denormalized table and avoid joins as possible.
Related
I am currently using PopSQL for my school project. Can someone help me with this problem?
The error message is self-explanatory. You need an index on the column that the foreign key refers to.
So, do create it:
create index idx_section_sectno on section(sectno):
Also, you might want to read the MySQL documentation for foreign key constraints:
MySQL requires indexes on foreign keys and referenced keys so that foreign key checks can be fast and not require a table scan.
Other conditions apply to forein keys that you want to be aware of.
I have a table in a SQL database that provides a "many-to-many" connection.
The table contains id's of both tables and some fields with additional information about the connection.
CREATE TABLE SomeTable (
f_id1 INTEGER NOT NULL,
f_id2 INTEGER NOT NULL,
additional_info text NOT NULL,
ts timestamp NULL DEFAULT now()
);
The table is expected to contain 10 000 - 100 000 entries.
How is it better to design a primary key? Should I create an additional 'id' field, or to create a complex primary key from both id's?
DBMS is PostgreSQL
This is a "hard" question in the sense that there are pretty good arguments on both sides. I have a bias toward putting in auto-incremented ids in all tables that I use. Over time, I have found that this simply helps with the development process and I don't have to think about whether they are necessary.
A big reason for this is so foreign key references to the table can use only one column.
In a many-to-many junction table (aka "association table"), this probably isn't necessary:
It is unlikely that you will add a table with a foreign key relationship to a junction table.
You are going to want a unique index on the columns anyway.
They will probably be declared not null anyway.
Some databases actually store data based on the primary key. So, when you do an insert, then data must be moved on pages to accommodate the new values. Postgres is not one of those databases. It treats the primary key index just like any other index. In other words, you are not incurring "extra" work by declaring one more more columns as a primary key.
My conclusion is that having the composite primary key is fine, even though I would probably have an auto-incremented primary key with separate constraints. The composite primary key will occupy less space so probably be more efficient than an auto-incremented id. However, if there is any chance that this table would be used for a foreign key relationship, then add in another id field.
A surrogate key wont protect you from adding multiple instances of (f_id1, f_id2) so you should definitely have a unique constraint or primary key for that. What would the purpose of a surrogate key be in your scenario?
Yes that's actually what people commonly do, that key is called surrogate key.. I'm not exactly sure with PostgreSQL, but in MySQL by using surrogate key you can delete/edit the records from the user interface.. Besides, this allows the database to query the single key column faster than it could multiple columns.. Hope it helps..
I just have a quick question. Can a table have it's only primary key as a foreign key?
To clarify. When I've been creating tables I sometimes have a table with multiple keys where some of them are foreign keys. For example:
create table Pet(
Name varchar(20),
Owner char(1),
Color varchar(10),
primary key(Name, Owner),
foreign key(Owner) referecnes Person(Ssn)
);
So now I'm wondering if it's possible to do something like this:
create table WorksAs(
Worker char(1),
Work varcahr(30),
primary key(Worker),
foreign key(Worker) references Person(Ssn)
);
This would result in two tables having the exact same primary key. Is this something that should be avoided or is it an ok way to design a database? If the above is not a good standard I would simply make the Work variable a primary key as well and that would be fine, but it seems simpler to just skip if it is not needed.
Yes, it's perfectly legal to do that.
In fact, this is the basis of IS-A relations ;)
Yes. Because of the following reasons.
Making them the primary key will force uniqueness (as opposed to imply it).
The primary key will presumably be clustered (depending on the dbms) which will improve performance for some queries.
It saves the space of adding a unique constraint which in some DBMS also creates a unique index
Yes, you might do so. But you need to be careful as foreign keys can have NULL values whereas Primary can't.
Sure. You can use this approach when mapping inheritance hierarchies using the Concrete Table Inheritance or Class Table Inheritance approach, see e.g. SQL Alchemy docs
For a Bridge Table I have the PK from 2 other tables. What are the pros and cons of making a PK field for the bridge table or making a composite/compound between the two fields.
I want to make sure I am following best practices.
Some Links I am reading over:
https://dba.stackexchange.com/questions/3134/in-sql-is-it-composite-or-compound-keys
http://social.msdn.microsoft.com/Forums/en-US/sqlgetstarted/thread/9d3cfd17-e596-4411-b3d8-66e0ec8bfdc7/
http://www.ben-morris.com/identity-surrogate-vs-composite-keys-in-sql-server
Composite primary keys versus unique object ID field
You have to enforce a unique constraint of some kind on the two foreign keys. The easiest way to do that is with a primary key constraint.
An additional, surrogate ID number isn't really useful. Some people use it because it makes foreign key constraints and joins to the "bridge" table easier to write. I think that, if you think it's hard to make a join using two integers, you shouldn't be working with databases in the first place.
In a LDM I recently made, I have an entity which has the following structure:
Building_ID (Primary Key, Foreign Key),
Plant_ID (Foreign Key),
Build_Year (Primary Key),
Size
I need to create a table in a SQL database using this design. The question I'm running into is how do I handle the primary keys here? Is it OK for a SQL table to have multiple primary keys? If the answer to this question is yes, then which column should act as the unique index? Should I create a new column to act as the unique index identifier?
Any SQL table for any relational database system (SQL Server, Oracle, Firebird, IBM DB2, Sybase etc.) I know can only ever have one primary key - after all, it's the primary key - there can only ever be one.
However, a primary key can be made up from multiple columns (called a "compound primary key"). There are downsides such as: all foreign key constraints from other tables also must specify all columns in the compound PK, thus making joining the tables a bit of a pain (since you need to specify all equality constraints for all columns included in the key in your JOIN).
Besides a primary key, you can also have multiple alternate keys - other column(s) that could also identify the row uniquely. Those make excellent candidates for e.g. indices, if those can help you speed up access to the table (but don't over-index your tables! Less is more)