I create a column as primary key , does this create indexes automatically ? or do i need to create indexes explicitly. i was under assumption that primary key also maintains indexes
In SQL Server creating a Primary key will create a Unique Clustered Index on that Column.
or more specifically from here
Note PRIMARY KEY constraints create
clustered indexes automatically if no
clustered index already exists on the
table and a nonclustered index is not
specified when you create the PRIMARY
KEY constraint.
This should clear out some air.
--creating a table without any primary key
CREATE table understanding_indexes
(P_Id int,
LastName varchar(255),
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
--
--checking for indexes
sp_helpindex understanding_indexes
OUTPUT
The object 'understanding_indexes' does not have any indexes, or you do not have permissions.
--ADDING A NOT NULL CONSTRAINT
ALTER TABLE UNDERSTANDING_INDEXES
ALTER COLUMN P_Id INTEGER
NOT NULL
--ADDING A PRIMARY KEY Constraint, can only be done on column which are not null.
ALTER TABLE UNDERSTANDING_INDEXES
ADD PRIMARY KEY (P_Id)
sp_helpindex understanding_indexes
OUTPUT
PK__understa__A3420A5702084FDA clustered, unique, primary key located on PRIMARY P_Id
Overall, As soon as you add a primary key constraint on a table, it automatically adds clustered indexes on the table.
This illustration is on SQL Server 2008 R2.
Related
I have a problem with something in SQL, let's see an example of database :
CREATE TABLE person( //Employee
pe_id PRIMARY KEY NOT NULL AUTO_INCREMENT,
pe_name VARCHAR(20),
pe_office VARCHAR(20)
);
CREATE TABLE project( //Mission
pr_id PRIMARY KEY NOT NULL AUTO_INCREMENT,
pr_name VARCHAR(20),
pr_status VARCHAR(15)
);
CREATE TABLE techno( //Programming language
te_id PRIMARY KEY NOT NULL AUTO_INCREMENT,
te_name VARCHAR(20)
);
CREATE TABLE job( //developer, manager, ...
jo_id PRIMARY KEY NOT NULL AUTO_INCREMENT,
jo_name VARCHAR(20)
);
I would like to assign persons on projects for a job using technos.
For example, Rob works as a developer and project manager on the projet #13 with AngularJS and HTML.
So I created this table :
CREATE TABLE assignment(
pe_id INT,
pr_id INT,
te_id INT,
jo_id INT,
as_days INT, //Days of work
PRIMARY KEY(pe_id, pr_id, tr_id, jo_id),
CONSTRAINT fk_as_pe_id FOREIGN KEY(pe_id) REFERENCES person(pe_id),
CONSTRAINT fk_as_pr_id FOREIGN KEY(pr_id) REFERENCES project(pr_id),
CONSTRAINT fk_as_te_id FOREIGN KEY(te_id) REFERENCES techno(te_id),
CONSTRAINT fk_as_jo_id FOREIGN KEY(jo_id) REFERENCES job(jo_id)
);
I would like to have the ability to assign a developer with somes technos to a project without knowing who exaclty, like this:
INSERT INTO assignment(pr_id,te_id,jo_id,as_days) VALUES(1,2,3,4); //No person!
We suppose that this values exists in project, techno and job tables.
But it seems that I can not insert this, probably because I do not define person's ID (which is in the primary key).
How can I do this ?
Hope I'm understandable :)
You solve this problem by not having this as a primary key. Primary keys cannot be NULL or, if they're composite primary keys, cannot contain NULL. Make it a unique index instead. Create an autonumber field for the primary key. I think this is better solution in your case
Primary Key:
Can be only one in a table
It never allows null values
Primary Key is unique key identifier and can not be null and must be unique.
Unique Key:
Can be more than one unique key in one table.
Unique key can have null values(only single null is allowed).
It can be a candidate key
Unique key can be null and may not be unique.
Maybe you should do this:
Before insert disable constraint:
ALTER INDEX fk_as_pe_id ON assignment
DISABLE;
After insert enable it:
ALTER INDEX fk_as_pe_id ON assignment
REBUILD;
Another alternate way is, if it is possible to alter table structure, just exclude pe_id from the composite primary key in assignment table
In my case, I have only 1 candidate may go with 1 job at the time so they are must be 2 primary key.
Then, a column is as JobApplicationId use for the table CandidateDetail as a foreign key.
Is that correct when I decide to set these 3 columns above as primary key or there are other ways to address my problem here?
CREATE TABLE Candidate(
CandidateId int identity primary key,
FullName nvarchar(50)
)
CREATE TABLE Job(
JobId int identity primary key,
JobTitle nvarchar(50)
)
CREATE TABLE JobApplication(
JobApplicationId int identity,
JobId int,
CandidateId int,
CreatedDate datetime,
primary key(JobApplicationId, JobId, CandidateId)
)
CREATE TABLE CandidateDetail(
CandidateDetailId int identity primary key,
JobApplicationId int,
[Description] nvarchar(300)
)
ALTER TABLE JobApplication ADD CONSTRAINT fk_JobApplication_Job FOREIGN KEY (JobId) REFERENCES Job(JobId)
ALTER TABLE JobApplication ADD CONSTRAINT fk_JobApplication_Candidate FOREIGN KEY (CandidateId) REFERENCES Candidate(CandidateId)
ALTER TABLE CandidateDetail ADD CONSTRAINT fk_CandidateDetail_JobApplication FOREIGN KEY (JobApplicationId) REFERENCES JobApplication(JobApplicationId)
Instead of a primary key with three columns you could just have JobApplicationId as the primary key and a unique constraint on JobId, CandidateId.
Otherwise, two rows with JobApplicationId=1, JobId=1, CandidateId=1 and JobApplicationId=2, JobId=1, CandidateId=1 would still be valid in terms of your current primary key approach, but would be invalid in terms of the business case.
From both a performance and usability perspective, a compound primary key can be a hassle and can create performance issues. Personally, I would choose JobApplicationId as the primary key (because this is an identity column and will be unique for each record). Then, if you need to constrain the table so that JobId and CandidateId are always unique (not allowing more than 1 record for any given candidate and the job they've applied for) then I would use a compound Unique Constraint.
However, I would suggest that you evaluate those requirements more closely because what if a candidate applies for the same position in a different time frame? It might stand to reason that having the same candidate applied to the same job more than once in that table might be valid data.
I have forgot create a primary key for my table. Now I want to update the DocDay_Id column and make it the primary key. How can I do it?
My code is below.
I tried this syntax but it is not correct.
ALTER TABLE DoctorDays
ALTER COLUMN DocDay_Id int IDENTITY(1,1) PRIMARY KEY NOT NULL
Create table DoctorDays
(
DocDay_Id int IDENTITY(1,1) NOT NULL,
Doc_Id int FOREIGN KEY REFERENCES Doctor(Doc_Id) NOT NULL,
Day_Id int FOREIGN KEY REFERENCES Dayss(Day_Id) NOT NULL
)
To create a clustered primary key on an existing table:
ALTER TABLE DoctorDays ADD CONSTRAINT PK_DocDays
PRIMARY KEY CLUSTERED (DocDay_Id);
To create a non clustered primary key on an existing table:
ALTER TABLE DoctorDays ADD CONSTRAINT PK_DocDays
PRIMARY KEY NONCLUSTERED (DocDay_Id);
FIDDLE DEMO HERE
See the document in below link:
Alter Statement Documents
I want to create tables in SQL Server 2008, but I don't know how to create composite primary key. How can I achieve this?
create table my_table (
column_a integer not null,
column_b integer not null,
column_c varchar(50),
primary key (column_a, column_b)
);
CREATE TABLE UserGroup
(
[User_Id] INT NOT NULL,
[Group_Id] INT NOT NULL
CONSTRAINT PK_UserGroup PRIMARY KEY NONCLUSTERED ([User_Id], [Group_Id])
)
Via Enterprise Manager (SSMS)...
Right Click on the Table you wish to create the composite key on and select Design.
Highlight the columns you wish to form as a composite key
Right Click over those columns and Set Primary Key
To see the SQL you can then right click on the Table > Script Table As > Create To
I know I'm late to this party, but for an existing table, try:
ALTER table TABLE_NAME
ADD CONSTRAINT [name of your PK, e.g. PK_TableName] PRIMARY KEY CLUSTERED (column1, column2, etc.)
For MSSQL Server 2012
CREATE TABLE usrgroup(
usr_id int FOREIGN KEY REFERENCES users(id),
grp_id int FOREIGN KEY REFERENCES groups(id),
PRIMARY KEY (usr_id, grp_id)
)
UPDATE
I should add !
If you want to add foreign / primary keys altering, firstly you should create the keys with constraints or you can not make changes. Like this below:
CREATE TABLE usrgroup(
usr_id int,
grp_id int,
CONSTRAINT FK_usrgroup_usrid FOREIGN KEY (usr_id) REFERENCES users(id),
CONSTRAINT FK_usrgroup_groupid FOREIGN KEY (grp_id) REFERENCES groups(id),
CONSTRAINT PK_usrgroup PRIMARY KEY (usr_id,grp_id)
)
Actually last way is healthier and serial. You can look the FK/PK Constraint names (dbo.dbname > Keys > ..) but if you do not use a constraint, MSSQL auto-creates random FK/PK names. You will need to look at every change (alter table) you need.
I recommend that you set a standard for yourself; the constraint should be defined according to the your standard. You will not have to memorize and you will not have to think too long. In short, you work faster.
First create the database and table, manually adding the columns. In which column to be primary key. You should right click this column and set primary key and set the seed value of the primary key.
To create a composite unique key on table
ALTER TABLE [TableName] ADD UNIQUE ([Column1], [Column2], [column3]);
CREATE TABLE UserGroup
(
[User_Id] INT Foreign Key,
[Group_Id] INT foreign key,
PRIMARY KEY ([User_Id], [Group_Id])
)
Naive question with the answer "No" , I believe, but still would like to ask.
Table_parent
pk_parent_surrogate
parent_natural_unique_key
Table_child
pk_child_surrogate
child_natural_NOT_unique
Is that true that the only possible declarative relationship among main database vendors is
pk_parent_surrogate ----------< pk_child_surrogate
and we can't have declarative constraint or foreign key in other words for pair
parent_natural_unique_key -------< child_natural_NOT_unique
My answer here is based on my MS SQL knowledge - although I believe the same answer is correct for ANSI standards as well, i'm not 100% sure...
YES - you CAN do this as long as you've got a unique constraint on the column in your parent table that you want to use as the anchor column for the key.
You can create a FOREIGN KEY constraint as part
of the table definition when you create a table.
If a table already exists, you can add a
FOREIGN KEY constraint, provided that the
FOREIGN KEY constraint is linked to an existing
PRIMARY KEY constraints or UNIQUE constraint in
another, or the same, table. A table can contain
multiple FOREIGN KEY constraints.
And as an example of this sort of key...
use tempdb
CREATE TABLE parent(
pk int identity primary key,
candidate_key int unique not null)
CREATE TABLE child(
pk int identity primary key,
join_key int references parent(candidate_key))
See here for more information.
Try code like this:
create table testunique (id int identity(1,1) primary key, otherid int)
go
create unique index ixOther on testunique(otherid)
go
create table testFK (id int identity(1,1) primary key, someid int)
go
alter table testFK add constraint fkTest foreign key (someid) references testunique(otherid)
Rob