I'm trying to create a default constraint here, but the system is generating a weird name for it. If I want to name it df_MY_TABLE_GUID or something, how could I specify that name be used?
ALTER TABLE MY_TABLE
ADD MY_GUID uniqueidentifier NOT NULL
CONSTRAINT uq_MY_TABLE_GUID UNIQUE (MY_TABLE_GUID)
DEFAULT NEWID() WITH VALUES
Just specify the constraint name with the full syntax, like the UNIQUE in your example:
ALTER TABLE MY_TABLE ADD MY_GUID UNIQUEIDENTIFIER NOT NULL
CONSTRAINT uq_MY_TABLE_GUID UNIQUE (MY_TABLE_GUID)
CONSTRAINT df_MY_TABLE_GUID DEFAULT NEWID() WITH VALUES ;
As a matter of routine, I always prefer and encourage to always name every single constraint I create, for the sake of easy reference latter on.
Related
In SQL I see several types of constraints, like PRIMARY KEY. NOT NULL, FOREIGN KEY, etc. But I stumbled on DEFAULT constraints now and I'm confused.
What does it prevent to happen? Will any query fail to adhere by it?
If no query will ever fail because of a DEFAULT constraint, is it really a constraint?
Thank you.
A confused developer
if a column is Not NULL
Like
CREATE TABLE mytable (val int NOT NULL)
and you try
INSERT NTO mytable VALUES (NULL)
You will get an error, as te database can't replace the NULL with a DEFAULT value, because it doesn't exist.
so it constraints the INSERT INTO
Check if the following answer helps you understanding about DEFAULT constraint.
https://stackoverflow.com/a/1213316/7056491
If a column has a DEFAULT constraint and the INSERT or UPDATE statement doesn’t provide the value for that column, MySQL will use the default value specified in the DEFAULT constraint.
So for table:
CREATE TABLE Defaults (
Age int,
Gender varchar(255) DEFAULT 'Female'
);
You can Insert:
INSERT INTO Defaults (`Age`) VALUES ('4');
Age will be 4, and Gender will be 'Female'.
The DEFAULT constraint does not RESTRICT the addition of any value nor does it enforce a rule like the other constraints, but it does provide proper predefined values to keep domain integrity intact.
From PostgreSQL document
To add a constraint, the table constraint syntax is used. For example:
ALTER TABLE products ADD CHECK (name <> '');
ALTER TABLE products ADD CONSTRAINT some_name UNIQUE (product_no);
ALTER TABLE products ADD FOREIGN KEY (product_group_id) REFERENCES product_groups;
To add a not-null constraint, which cannot be written as a table
constraint, use this syntax:
ALTER TABLE products ALTER COLUMN product_no SET NOT NULL;
Why the word after ADD differ for constraints of different kinds?
Why does unique constraint has a more generic ADD CONSTRAINT some_name than check and foreign key?
Why is not null constraint specified by ALTER COLUMN, instead of ADD CONSTRAINT some_name NOT NULL (col_name)?
Is there inconsistency in PostgreSQL syntax of adding constraints to a table?
Does this belong to the SQL standard?
The word after ADD differs so the database knows what you mean. E.g. CHECK introduces a generic boolean condition; UNIQUE is followed by a list of column names; FOREIGN KEY is followed by a column name, REFERENCES, and a target table/column. Without these keywords it would be ambiguous which kind of constraint you mean.
The CONSTRAINT constraint_name syntax is not limited to unique constraints. See the definition of column_constraint and table_constraint in https://www.postgresql.org/docs/10/static/sql-createtable.html; both allow an optional leading CONSTRAINT constraint_name to name the constraint.
As for NOT NULL, see https://www.postgresql.org/docs/10/static/ddl-constraints.html#id-1.5.4.5.6:
A not-null constraint is always written as a column constraint. A not-null constraint is functionally equivalent to creating a check constraint CHECK (column_name IS NOT NULL), but in PostgreSQL creating an explicit not-null constraint is more efficient. The drawback is that you cannot give explicit names to not-null constraints created this way.
I assume not-null constraints are a special case internally, allowing for better optimization than a generic CHECK constraint, which can use any boolean expression.
We cant use add constraint syntax for Not Null. You have to use modify column syntax to add not null
eg.
alter table modify ( not null);
I have just inherited a database from another developer, and I have looked through the sys.objects table, filtering by constraints.
What does DF__role_sett__custo__4589517F mean - mainly the ID at the end of the string?
I know that DF_role_sett_custo means default constraint of role_setting_customer, but am not sure of the last ID 4589517f.
If you don't name a constraint when it is created, SQL Server will assign it a random name based on the table and column. It appends a random number so that it doesn't clash with existing constraint names.
In almost all cases, it is best to name a constraint when it is created. It's then easier to refer to the constraint by name in other T-SQL statements.
For instance, in the following create statement
CREATE TABLE dbo.some_table(
some_field INT NOT NULL DEFAULT(5)
);
The default constraint will be assigned a random name. In this statement:
CREATE TABLE dbo.some_table(
some_field INT NOT NULL CONSTRAINT DF_some_table_some_field DEFAULT(5)
);
The default constraint will have the name you assigned to it (i.e. DF_some_table_some_field).
I am trying to add a constrain to a database table that I want to modify. I want to add a constraint on a column so it references a primary key of another tables. Easy enough, I just have to add a foreign key constraint. The problem is that the column already has some values that are null or something that is not part of the table I will be referencing.
My question is how do I add a constraint that references a primary key but can also accept null values (the primary key always has a value) and how to ignore the existing values so far. Is it possible? If the second part is not, I am thinking I could always write a script that updates all the nonsense values (they have a format of sort if that I can reg ex) to null so they only thing I have to figure out is how to add a foreign key constraind that also accepts null values
Firstly, there's nothing stopping you from adding a referential constraint on a column that has NULLs - foreign key constraints are only enforced for non-NULL values.
Secondly, if there are existing values that do not exist in the parent table, and you can't fix them, you do have the option in Oracle to make the constraint only validated for newly inserted or updated rows, using the NOVALIDATE option, e.g.
ALTER TABLE x ADD CONSTRAINT fk FOREIGN KEY (id) REFERENCES parent (id) NOVALIDATE;
The only downside to using the NOVALIDATE option is that the query optimizer will not rely on the FK constraint, and will execute queries with the assumption that there may be rows that do not have a matching parent row.
It would be a good idea if you are able to fix the missing values, afterwards to alter the constraint to VALIDATE.
Primary keys can not contain NULL values in all databases (proof)
To add a foreign key you should do something like:
ALTER TABLE table1
ADD CONSTRAINT fk_table1_2
FOREIGN KEY (column1)
REFERENCES table2(column2);
There appears to be at least two ways to add a default constraint using straight T-SQL. Am I correct that the only difference between the two below is that the second method specifically creates a name for the constraint, and the first method has one generated by SQL Server?
ALTER TABLE [Common].[PropertySetting] ADD DEFAULT ((1)) FOR [Active];
ALTER TABLE [Common].[PropertySetting] ADD CONSTRAINT [DF_PropertySetting_Active) DEFAULT ((1)) FOR [Active];
Pretty much, yes for an ALTER TABLE
You can add a columnn with default in one step for CREATE or ALTER too.
ALTER TABLE dbo.TableName
ADD bar varchar(100) CONSTRAINT DF_Foo_Bar DEFAULT ('bicycle');
ALTER TABLE dbo.TableName
ADD bar varchar(100) DEFAULT ('bicycle');
As you noted, the system generates a name if one is not supplied. CONSTRAINT constraint_name is optional says MSDN. The same applies to any column or table CONSTRAINT
If the column was already created, and you only want to add a (named) DEFAULT constraint, then use:
ALTER TABLE dbo.TableName
ADD CONSTRAINT DF_Foo_Bar DEFAULT 'bicycle' FOR FieldName;
To have the system generate the DEFAULT constraint name (which will be of the form DF_{TableName}_{Column}_{8RandomChars}, e.g. DF_TableName_FieldName_12345678) then omit the CONSTRAINT <name> part, like so:
ALTER TABLE dbo.TableName
ADD DEFAULT 'bicycle' FOR FieldName;