Navicat does not show primary keys which are also foreign key on table report as foreign keys. Why?
I gave the image explaining the situation:
A foreign key is a constraint that applies only to the referencing table. In your case, the translate_talent_id field has a foreign key constraint that references another field of another table.
On the other hand, translator_id is probably referenced by foreign keys in other tables. However, such foreign keys won't appear (or have any effect) on the the referenced table (trl_translator in this case). That is why your MySQL client is not showing any foreign keys on translator_id.
Related
Tell me please either should I to give name the foreign key?
CONSTRAINT constraint_name
FOREIGN KEY foreign_key_name (columns)
REFERENCES parent_table(columns)
ON DELETE action
ON UPDATE action
I can do some manipulations with the constraint by it's name, but what I can do with the foreign key name? Give me some examples please.
As the documentation explains:
MySQL requires indexes on foreign keys and referenced keys so that
foreign key checks can be fast and not require a table scan. In the
referencing table, there must be an index where the foreign key
columns are listed as the first columns in the same order. Such an
index is created on the referencing table automatically if it does not
exist. This index might be silently dropped later, if you create
another index that can be used to enforce the foreign key constraint.
index_name, if given, is used as described previously.
In other words, what you are providing is not a "foreign key name" but a "(foreign key) index name".
Having a name for an index is useful for tracking that index.
To be honest, though, I don't provide such names. I would much rather explicitly declare an index on the foreign keys, rather than have the database do it for me.
(Note: Most databases do not automatically create an index when a foreign key is declared.)
Yes it is.
If you want to alter or drop constraint in future,then it is possible using name only.
DROP FOREIGN KEY constraint_name;
You can check here.
I have tables named 'studentdetails', 'class', 'obtainedmarks', and 'subject' in a database.
I have a primary key named 'STUDENTID' in table 'studentdetails' which is connected to other tables as a foreign key.
I want to make one more primary key in the same table 'studentdetails' taking three columns ('STUDENTID','CLASS','ROLLNO') so that I can use the foreign key relation to the three columns of 'obtainedmarks' table.
How can I establish such relation as we can make only one primary key in a table?
Mysql certainly allows foreign key relationships to span multiple columns. In fact there is even a full example in the manual. Thus your relationship might look something like
CONSTRAINT fk_multi FOREIGN KEY (`studentid`, `class`,`subject`)
REFERENCES other_table (`studentid`, `class`,`subject`)
ON DELETE CASCADE ON UPDATE CASCADE,
Note however that what goes into the REFERENCES section is the names of the columns and not the name of the index on that other table.
Getting onto the Primary Key problem, it's true that you can only have one primary key per table. But there's nothing to prevent you from creating a composite unique key on the three columns that are referenced.
I know there have been myriads of questions concerning primary and foreign keys. Looking through them, I cannot seem to find a simple answer to my question. My understanding of primary and foreign keys is that a foreign key is a column designated in a child table that refers back to a primary key as a column in a parent table. Is that correct, or do I have it backwards? If that is indeed correct, I am trying to find out why I am having difficulty creating a foreign key in a child table as such:
salesorders.sonumber (pk) < customer.sonumber (fk)
I am using Navicat with MariaDB (same as MySQL) and the error I get is:
1452 - Cannot add or update a child row: a foreign key constraint fails ('customer_orders','#sql7a8_3'; CONSTRAINT 'sonumber' FOREIGN KEY ('sonumber') REFERENCES 'salesorder' ('sonumber') ON DELETE NO ACTION ON UPDATE CASCADE)
Customer_orders is the database name. I am naming the foreign key 'sonumber' which is the same as the column name in the child table (customer) and the parent table (salesorders). Is that incorrect? Should I give the foreign key another name?
gitpicker
The foreign key should have its own unique name, yes. sonumber_fk or something similar is a simple naming schema.
You also need to make sure that every entry in the Foreign Key column has a corresponding entry in the Referenced table, otherwise you will not be able to create the foreign key.
If two tables have foreign keys to the same primary key of another table, does that mean that two indexes are created for those foreign keys or do they use the same index (the primary key index?)
Two indexes will be created; one for each foreign key (making for a total of three including the index on the primary key column). It should be noted that this is not true of other database systems which require you to explicitly add an index on a foreign key. MySQL's innodb requires that an index be created when creating a foreign key relation.
See Foreign Keys for more.
In my table I have a foreign key which is also a unique key.
How can I remove the unique Key without removing the foreign key?
When I do :
ALTER TABLE affaire DROP KEY contact_client_id;
I am getting a : ERROR 1553 (HY000): Cannot drop index 'contact_client_id': needed in a foreign key constraint
I don't think that this is possible because foreign keys must be on indexes.
MySQL requires indexes on foreign keys and referenced keys so that
foreign key checks can be fast and not require a table scan. In the
referencing table, there must be an index where the foreign key
columns are listed as the first columns in the same order. Such an
index is created on the referencing table automatically if it does not
exist. This index might be silently dropped later, if you create
another index that can be used to enforce the foreign key constraint.
index_name, if given, is used as described previously.