In mysql, say I have:
create table users(
id not null
)
Let's say I need to make id as a primary key. What is the difference between:
create table users(
id primary key not null
)
and
create table users(
id not null
primary key (id))
and
create table users(
id not null
constraint pk primary key (id))
I've been searching a lot for the meaning of constraints in this context, but I only find how to use them, not what it actually is.
A primary key is both not null and unique. So, this is very, very similar to a primary key:
create table users (
id int not null unique
)
The one additional feature of a primary key is that it is usually also the clustered index for the table.
A primary key declaration is a constraint. It has the following properties:
The columns are not null.
The columns are unique.
Only one primary key declaration is allowed per table (although multiple columns can be in the primary key).
In addition, the primary key columns often form a clustered index.
Except for the third condition, it is possible to declare these using multiple constraint declarations.
I'm beginning with MySQL, trying to set up a 2-columns primary key, I'm using phpmyadmin.
I managed to somehow mark two columns as a primary key (this is what i have right now, the primary columns are underligned) but they seem to act as two separate primary key, I can't add rows with the same ID and different region, or reversibly the same region and different ID.
What should i fix ? thanks !
If you run SHOW CREATE TABLE you will most likely see the following:
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
region VARCHAR,
....
PRIMARY KEY (id),
UNIQUE KEY somename (id,region)
So what has been created for you is a unique key. A unique key can be used as primary key, but you will have to get rid of your other primary key id.
This can be done by:
ALTER TABLE your_table_name DROP PRIMARY KEY;
Since I do not know all your specs, test the result and see if all the desired behavior is still in place.
I previously created a table using the following SQL code:
CREATE TABLE myTable (
id_A BIGINT NOT NULL,
id_B INT NOT NULL,
some_info varchar(255),
some_info2 varchar(255),
PRIMARY KEY (id_A)
)
In the previous table created, both id_A and id_B would be unique values. I understand that id_A is forced to be unique by the
PRIMARY KEY (id_A) code, which is perfect (which also indexes it), however id_B isn't
Here is where I am confused. id_B will also be unique, however I am not sure how to force it to be unique in the table,and how to make the database create an index for it so that future queries that use SELECT on this table will have good preformance. I know I can't have two PRIMARY KEYS:
PRIMARY KEY (id_A)
PRIMARY KEY (id_B)
How might I go about making an index for id_B as well so that future queries happen efficiently?
You can add a UNIQUE INDEX on the column. While a table can only have one PRIMARY KEY, it can have as many UNIQUE INDEXes as you might need.
ALTER TABLE myTable
ADD UNIQUE INDEX `UI_myTable_idB` (`id_B` );
You can use the MySQL UNIQUE KEY for the column. This will force the values in the id_B column to be unique, but not use it as the primary key column. You can achieve it with this statement:
CREATE TABLE myTable (
id_A BIGINT NOT NULL,
id_B INT NOT NULL,
some_info varchar(255),
some_info2 varchar(255),
PRIMARY KEY (id_A),
UNIQUE KEY (id_b))
This will automatically index the column like you want it to. Primary keys and unique keys in a MySQL table are essentially the same, except a primary key cannot be NULL, and there can be only one primary key column (or combination of primary key columns). There can be any number of unique key columns.
I am learning about sql and saw the following:
create table order_items
( orderid int unsigned not null,
isbn char(13) not null,
quantity tinyint unsigned,
primary key (orderid, isbn)
);
The line in question is: primary key (orderid, isbn)
How can you have two columns set as primary keys? How does this work? I understand having one column as a primary key like your social security, it's unique.
A primary key is one or more columns that have the following two properties:
The values are unique.
The columns are not null.
This simply means that pairs of values of those two columns uniquely identify a row. A table can only have one primary key.
In my opinion, I prefer auto-incremented numeric primary keys, with a secondary (unique composite) index for these two columns. One reason is foreign key references, which I find easier to maintain with a single column.
I am very confused right now, maybe you can help me to understand the problem better regarding the question that can a table have two primary keys if yes then how ? And if no, then why?
You ask if you can have more than one primary key field and you most certainly can. You can have only one primary key, but that can consist of as many columns as you need to uniquely identify your rows.
Use something like this when you are creating your table:
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
where P_Td and LastName are columns in your table.
If you think you want more than one primary key, then the answer is "not really." You can have only one primary key. However, you can have as many indexes as you want that have a unique constraint on them. A unique index does pretty much the same thing as a primary key.
for example :-
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
)
Note: In the example above there is only ONE PRIMARY KEY (pk_PersonID). However, the value of the pk_PersonID is made up of two columns (P_Id and LastName).
No You cannot have two primary keys in one table, but you can have composite primary key
Because Primary key is an identity to the row and there can't be two IDs against a row.
A table can have many keys but by convention only one key per table is designated a "primary" one. Typically this may be the key deemed to be the "preferred" identifier for the information in that table or it may be a key singled out for some other specific purpose by the table's designer.
In principle whatever function or property you associate with the key designated "primary" could just as well be associated with any other key as well. Therefore for many practical purposes you could designate more than one such "primary" key if you so choose - but only if the limitations of any particular DBMS permit.
Under the relational model of data all keys are equal and there is no special function given to primary keys (in fact the relational usage of the term primary key originally referred to any and all keys of a relation and not just one key). Unfortunately many DBMSs don't respect this principle and may limit certain features to one and only one key of a table, making it necessary to be selective about which key gets designated as primary. So the answer to your question ought to be YES in principle. When you need to achieve it in some particular SQL DBMS the actual answer is: it depends.
You can only have 1 primary key - the range of keys that could all potentially be the primary key can be referred to as candidate keys. The one you select is the primary key, the other alternative keys can be implemented as unique constraints / indexes.
So whilst there is only 1 primary key, you can still ensure primality of other fields / combination of fields using the unique constraint / index.
No.The table have only on primary key. But that primary key can contain multiple field. Means when you create table and when you mention primary key, you can add more then one column which you want to.
for example
CREATE TABLE table_name ( col1 Datatype , col2 Datatype,col3 Datatype, col4 Datatype, PRIMARY KEY (col1,col2,col3) )
By this way you can add primary key in single table
On a table you can make indexes, which allow the internal database engine to process the contents of the affected columns (1 to many) for easy lookup. Because the engine is at that point already evaluating and sorting the contents of the fields, it can also easily ensure uniqueness of the values. Thus an index can span 1 to many rows, and optionally also be unique.
A primary key is a theoretically optional, though in practice mandatory, marker on a specific index that it's the eternally unique way of referencing a specific row in the table. It's usually either a GUID or an auto-increment integer (identity in SQL Server). The primary key itself is unique for any given table, and enforces a unique constraint by definition, but can optionally span multiple rows (a spanned index/key).
You could for example have a junction table containing only 2 fields, which are both foreign keys, and together form the primary key/index of the table.
You Can try FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table.
No.You cannot use more than 1 primary key in the table.for that you have composite key which is combination of multiple fields.
As you cannot define more than one column as Primary Key as below
create table test1 (col1 numeric(10) primary key, col2 numeric(10) primary key
,col3 numeric(10) primary key, col4 numeric(10))
It needs to be a composite key. Yes, we can have more than one column as primary key to solve some business requirements. Primary Keys ensures that the column(s) will not have duplicate values , Null in the table.
Below is the SQL to create a table with Composite Primary Key
CREATE TABLE track(
col1 numeric(10) , col2 numeric(10) ,col3 numeric(10) primary key, col4 numeric(10),
PRIMARY KEY (col1,col2,col3)
)
CREATE TABLE track(
col1 numeric(10) , col2 numeric(10) ,col3 numeric(10) , col4 numeric(10),
PRIMARY KEY (col1,col2,col3)
)