I know that the following code says that every node with the label City has a unique value for the location property.
CREATE CONSTRAINT ON (c:City)
ASSERT c.location IS UNIQUE;
So this code forbids me to have two cities with the same name in one country, e.g. there can be only one London in England. Now I need to turn the constraint off. How can I do that?
There is no way to "switch off" a constraint. You have to drop it using the DROP CONSTRAINT command. This will delete the constraint.
Make sure you have the constraint name before dropping it. If you are not sure about the constraint name, then you can list all constraints using the SHOW CONSTRAINTS command. It is always a good practice to specify a constraint name when creating it. In the below example, I specified the constraint name as constraint_city:
CREATE CONSTRAINT constraint_city ON (c:City)
ASSERT c.location IS UNIQUE;
Then, to drop the constraint:
DROP CONSTRAINT constraint_city;
Related
I am currently learning SQL, and I have a physical data model I need to implement in code. However, during constraint creation, the numbers appearing next to FK and U started confusing me immensely. Consider the diagram. EDIT: Added the full physical model.
I know that when the matter is Primary Keys, we must have a single PK Constraint that's all the columns marked as PK. However, when the thing is FK or Unique constraints, I'm not so sure myself.
Let's assume I want to create the FK constraints for the table Opcao.
Should I create a single constraint for multiple columns, referencing their respective columns like this:
ALTER TABLE MySchema.Opcao ADD CONSTRAINT [FK_SUPERKEY] FOREIGN KEY ([prova], [aluno], [pergunta], [dataRealizacao])
REFERENCES MySchema.Integra([prova], [aluno], [pergunta], [dataRealizacao]);
Or create a constraint for each column, like this:
ALTER TABLE MySchema.Opcao ADD CONSTRAINT [FK_OPCAO_PROVA] FOREIGN KEY ([prova])
REFERENCES MySchema.Integra([prova]);
ALTER TABLE MySchema.Opcao ADD CONSTRAINT [FK_OPCAO_ALUNO] FOREIGN KEY ([aluno])
REFERENCES MySchema.Integra([aluno]);
ALTER TABLE MySchema.Opcao ADD CONSTRAINT [FK_OPCAO_PERGUNTA] FOREIGN KEY ([pergunta])
REFERENCES MySchema.Integra([pergunta]);
ALTER TABLE MySchema.Opcao ADD CONSTRAINT [FK_OPCAO_DATAREALIZACAO] FOREIGN KEY ([dataRealizacao])
REFERENCES MySchema.Integra([dataRealizacao]);
Would the Unique constraints follow the same logic? How do I know when to do one or the other?
You want to make a foreign key consisting of three columns which have to match all the three columns in the referenced table?
Then you should use in my oppinion on constraint for the three columns, because its the semantic you want to tell.
The one constraint for each column approach has the same effect, but you have to think a little to get the intension.
Some other tips: I don't get the semantic of the schema because i don't know the language the entities are named in. It would be easier if they were named in english. One thing i saw is the pergunta column which is duplicated and needs to be consistent in opcao, Integra und Pergunta table, this may lead to problems.
I generally helped me to always make an artifical auto increment primary key for every table (even the join tables for n to m relations), and always reference this artificial key. Then you have less problems (with case insensitivity for example) and the schema is in my oppinion easier to understand.
Let's assume I have a table called boxes with the box_id attribute as the PK.
There are two other tables. The first one is red_boxes and the second blue_boxes.
I have added a constraint to the red_boxes table
ALTER TABLE red_boxes
ADD CONSTRAINT fk_box_id
FOREIGN KEY (box_id)
REFERENCES boxes (box_id);
Now, I would like to add a constraint to the blue_boxes table. The SQL structure would look like the following, if I did not add the constraint already to the the red_boxes. The obvious way to fix this is to name a new constraint differently e.g. fk_box_id2, but is this is a good way? Am I supposed to somehow re-use the previous constraint, or this is not possible, why?
ALTER TABLE blue_boxes
ADD CONSTRAINT fk_box_id
FOREIGN KEY (box_id)
REFERENCES boxes (box_id)
Each constraint is separate and requires a unique name. My recommendation is to use the source and destination table names, for example fk_red_boxes_boxes and fk_blue_boxes_boxes. This way you can easily identify where they come from and where they go to.
If you have underscores in your table names, you might want to come up with a modified convention that you can easily understand at a glance. For example, a double underscore: fk__blue_boxes__boxes and fk__red_boxes__boxes.
I am completely new to sql (have couple of days to get to know it) and have following question:
Here is a syntax for constraints:
While creating the table, I have this kind of line:
CONSTRAINT smth UNIQUE(name)
I understand that it puts constraint on column name for it to be unique but what is smth for? Why do we need to name the constraint? Is it used anywhere?
You name it (as with many things) so that you can perform maintenance in your database easily.
See ALTER TABLE ... DROP CONSTRAINT and note that you have to supply the name of the constraint there.
Also, it's helpful if the constraint is violated:
An optional name for a column or table constraint. If the constraint is violated, the constraint name is present in error messages, so constraint names like col must be positive can be used to communicate helpful constraint information to client applications.
Constraints have names.
It is useful. Just imagine:
when you need to drop a constraint
when you list constraints on a object
when a constraint fails, it will show name in error message.
CONSTRAINT constraint_name UNIQUE(column_name)
column_name: to which you are applying constraint
constraint_name: name of the constraint, which you are applying to column_name
It is used to identify the UNIQUE(present declared) constraint and can able to delete if not needed
Need to add some sort of constraint on my table column. I need to make sure that the user does not enter, manually or otherwise, the same text in more than one row of the table. How can I achieve this?
You'll want to use some type of unique constraint or, of course, the primary key would work as well. For example:
ALTER TABLE MyTable ADD CONSTRAINT UK_MyUniqueConstraint UNIQUE (MyColumn)
There are other methods of creating this discussed on SE as well.
I would like to design a table named arguments whose an attribute name is linked to another attribute name in a table called names.
I see two ways to express it in SQL:
by creating a constraint on the table:
CREATE TABLE names ( name text UNIQUE,
short text UNIQUE,
comment text);
CREATE TABLE arguments ( name text UNIQUE,
comment text,
FOREIGN KEY (name) REFERENCES names (name));
by qualifying the attribute on-the-fly:
CREATE TABLE names ( name text UNIQUE,
short text UNIQUE,
comment text);
CREATE TABLE arguments ( name text UNIQUE REFERENCES names (name),
comment text);
I would like to know:
if one of the two is commonly known as better than the other, and
if it can have consequences that I should be aware of.
Thank you for your help.
These are just different syntax for the same end result.
Either is appropriate, but the former is a more common style in my experience. This may simply be to allow the human mind to more easily digest all the information. First describe the data, second describe how it relates to the rest of the world.
One comment I would make though, is that is more common to have IDs as unique identifiers and references. This allows you to change the Value in the Name field without changing it's Identity and breaking Referential Integrity. There are databases that can cascade such changes and update all occurrences of the Name, but in general it's considered cleaner to have Identifiers that are Separate from the Data.
While the first option is known as out-of-line constraint declaration and the second option is in-line, both of them are functionally same.
What would be better is to assign a name to the foreign key constraint. If you have a name, you can selectively enable and disable the constraint if required.
Create table
CREATE TABLE arguments
(
name text UNIQUE,
comment text,
constraint arguments_fk FOREIGN KEY (name) REFERENCES names (name)
);
Disable constraint
ALTER TABLE arguments NOCHECK CONSTRAINT arguments_fk;
Enable constraint
ALTER TABLE arguments CHECK CONSTRAINT arguments_fk;
This is for SQL Server. Oracle has equivalent commands.
Use a foreign key. If you later find you have a performance problem (a measurable problem), then you can change it up to be different.
Keep things simple at first and get the product into the users' hands as fast as possible. Don't optimize things that you can't prove need it.