I want to create a table department:
COLUMN NAME DATATYPE SIZE CONSTRAINT
dept_id number 4 Primary key
prod_id number 4 Foreign key
I tried this:
CREATE TABLE Department(
dept_id number(4) primary key,
prod_id number(4) foreign key);
It shows error. How can I add a foreign key constraint to this table?
A foreign key defines a relationship between your table DEPARTMENT and another table with a primary key. It means, you cannot create a row in DEPARTMENT with a PROD_ID of 1234 unless there is a pre-existing row in the designated parent table with a value of 1234 as its primary key.
So do you have such an existing parent table? if so you need to include its name in the foreign key definition. Otherwise you must create it.
Let's say the parent table is PRODUCT:
create table product (
prod_id number(4) primary key
, name varchar2(32) not null
);
Then you can create DEPARTMENT with a foreign key like this:
CREATE TABLE Department(
dept_id number(4) primary key,
prod_id references PRODUCT );
Yep, that's all the syntax you need: it automatically creates a column PROD_ID with the same datatype and precision as the primary key column of the referenced table. More verbose syntax is available. Read the Oracle SQL documentation to find out more.
I assume that the other table is named other_table_name and that it contains a primary key named prod_id.
CREATE Department (
dept_id number(4) primary key,
prod_id number(4) REFERENCES other_table_name (prod_id)
);
or a different syntax
CREATE Department (
dept_id number(4) primary key,
prod_id number(4)
...
CONSTRAINT fk_prod_id
FOREIGN KEY (prod_id)
REFERENCES other_table_name (prod_id)
);
Related
For the Epms table, I want to have only 3 columns (departure_City_Id, arrival_City_Id, Emp_number). However, when I come to insert value, it says that I have 4 columns. The extra one for City_Id. All I want is to avoid having City_Id column in this table. I declare it because I need it just as FK.
CREATE TABLE City (
City_Id char(3),
state varchar(30),
Primary key (City_Id)
);
create table Emps (
Emp_number varchar(30) primary key,
City_Id char(3),
departure_City_Id char(3),
arrival_City_Id char(3),
FOREIGN KEY (City_Id)
REFERENCES City(City_Id),
FOREIGN KEY (City_Id)
REFERENCES City(City_Id)
);
This is pretty straightforward, just specify e.g. , FOREIGN KEY (arrival_city_id) REFERENCES city(city_id) and you'll get an index.
CREATE TABLE emps (
emp_number varchar(30) NOT NULL,
city_id char(3) NOT NULL,
departure_city_id char(3) NOT NULL,
arrival_city_id char(3) NOT NULL,
PRIMARY KEY (emp_number),
KEY city_id (city_id),
KEY departure_city_id (departure_city_id),
KEY arrival_city_id (arrival_city_id),
CONSTRAINT emps_ibfk_1 FOREIGN KEY (city_id) REFERENCES city (city_id),
CONSTRAINT emps_ibfk_2 FOREIGN KEY (departure_city_id) REFERENCES city (city_id),
CONSTRAINT emps_ibfk_3 FOREIGN KEY (arrival_city_id) REFERENCES city (city_id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
" I want is to avoid having City_Id column in this table. I declare it because I need it just as FK."
But you don't need it. The foreign key columns are the arrival and departure columns, and those are the ones you must reference in the constraint declarations:
create table Emps (
Emp_number varchar(30) primary key,
departure_City_Id char(3),
arrival_City_Id char(3),
FOREIGN KEY (departure_City_Id)
REFERENCES City(City_Id),
FOREIGN KEY (City_Id)
REFERENCES City(arrival_City_Id)
);
Naming constraints is optional but you may find it helpful when debugging foreign key failures, especially when a table has multiple constraints referencing the same parent key.
How to link the MgrId in ManagerProject to EmpId in the Employee table ?
This is wat I tried :
CREATE TABLE Employee(EmpId varchar2(5),
EmpName varchar2(25),
DeptId varchar2(3),
Salary Number(8),
Constraint PK_addn primary key (EmpId, DeptId),
Constraint fk_Department foreign key (DeptId) references Department (DeptId));
But the second table failed to be created :
CREATE TABLE ManagerProject(ProjId varchar2(4),
MgrId varchar2(5),
StartDate Date,
EndDate Date,
Constraint fk_managerproject foreign key (MgrId) references Employee (EmpId),
Constraint PK_Managerproject Primary key(ProjId, MgrId, StartDate));
It displays
ORA-02270: no matching unique or primary key for this column-list
The error message says that you are trying to create a FK referencing a column on which there is no Unique or Primary Key constraint.
Assuming that you don't want to add the column DeptId to ManagerProject, you need to add a unique key on employee:
alter table Employee add constraint empId_UK unique ( empId)
But this strongly depends on what your schema should be.
If you want to add the column DeptId to ManagerProject, you will need to edit your FK to both use EmpId and DeptId in referencing employee.
I have two table students and studentsprofilepic
'username' from students is unique key of the table
it is referenced as foreign key for the 'studentsprofilepic' table
the DDL for the tables are
CREATE TABLE students (
id NUMBER,
username VARCHAR2(30),
password VARCHAR2(30),
firstname VARCHAR2(30),
lastname VARCHAR2(40),
email VARCHAR2(300),
dob VARCHAR2(20),
alt_email VARCHAR2(300),
street_address VARCHAR2(50),
address_2 VARCHAR2(50),
city VARCHAR2(30),
state VARCHAR2(30),
zip VARCHAR2(10),
country VARCHAR2(60),
telephone VARCHAR2(10),
CONSTRAINT student_id_pk PRIMARY KEY (id),
CONSTRAINT student_username_uk UNIQUE (username)
);
CREATE TABLE studentsprofilepic (
id NUMBER,
photo_id NUMBER,
photo BLOB,
PRIMARY KEY (photo_id),
FOREIGN KEY (username) REFERENCES students (username)
);
YES, The foreign key column establishes a direct relationship with a primary key or unique key column (referenced key) usually in another table:
CREATE TABLE BOOK(
BNAME VARCHAR2(10)NOT NULL UNIQUE,
BTYPE VARCHAR2(10));
CREATE TABLE BOOKS_AUTH(
A_ID INT NOT NULL,
BNAME_REF VARCHAR2(10) NOT NULL,
FOREIGN KEY (BNAME_REF) REFERENCES BOOK (BNAME));
SQLFIDDLE DEMO
Yes, why not. It is possible to reference a UNIQUE constraint in a FOREIGN KEY.
You could have a Primary key and an Unique key, and you would like to validate both.
Yes, you can reference a column (or columns) governed by either a primary key constraint or a unique constraint.
The problem with your table "studentsprofilepic" is that your foreign key tries to use the column "studentsprofilepic"."username", but that column doesn't exist.
create table studentsprofilepic(
id number,
photo_id number,
photo blob,
-- Add the "username" column.
username varchar2(30) not null,
primary key (photo_id),
foreign key (username) references students (username)
);
Also, ask yourself "What's the point of "studentsprofilepic"."id"?" It's not the primary key. It's not a foreign key. It doesn't seem to serve any purpose besides letting you say, "Hey, my table has a column named 'id'!" That's a questionable feature.
Think about adding more not null constraints.
Table has been created in system this way
CREATE TABLE INSTANCES
(
DM INTEGER NOT NULL,
INSTANCEID VARCHAR2(512) NOT NULL,
INSTANCENAME VARCHAR2(64) NOT NULL UNIQUE,
HOSTNAME VARCHAR2(32) NOT NULL,
CONSTRAINT PK_INSTANCES PRIMARY KEY (INSTANCEID, HOSTNAME)
);
The new crete table statement is as below:
CREATE TABLE INSTANCES
(
DM INTEGER NOT NULL,
INSTANCEID VARCHAR2(512) NOT NULL UNIQUE,
INSTANCENAME VARCHAR2(64) NOT NULL UNIQUE,
HOSTNAME VARCHAR2(32) NOT NULL,
CONSTRAINT PK_INSTANCES PRIMARY KEY (INSTANCEID, HOSTNAME)
);
The differnce is INSTANCEID has UNIQUE in it. How do i Alter the table? I used the below statement and it did not work for me.
ALTER TABLE INSTANCES ADD CONSTRAINT ab UNIQUE ( INSTANCEID);
It gave an error:
ALTER TABLE INSTANCES ADD CONSTRAINT ab UNIQUE ( INSTANCEID)
Error report:
SQL Error: ORA-02261: such unique or primary key already exists in the table
02261. 00000 - "such unique or primary key already exists in the table"
*Cause: Self-evident.
*Action: Remove the extra key.
Please help me to Alter the table as required above. Thanks!
Here is the output of SELECT con.constraint_name, col.column_name, con.constraint_type
FROM user_cons_columns col
JOIN user_constraints con ON (col.constraint_name = con.constraint_name)
WHERE col.table_name = 'INSTANCES';
"CONSTRAINT_NAME","COLUMN_NAME","CONSTRAINT_TYPE"
"SYS_C0016531","DM","C"
"SYS_C0016532","INSTANCEID","C"
"SYS_C0016533","INSTANCENAME","C"
"SYS_C0016534","HOSTNAME","C"
"PK_INSTANCES","HOSTNAME","P"
"PK_INSTANCES","INSTANCEID","P"
"SYS_C0016536","INSTANCENAME","U"
You have already stated that INSTANCEID is supposed to be UNIQUE, so a constraint has been created.
CREATE TABLE INSTANCES
(
DM INTEGER NOT NULL,
INSTANCEID VARCHAR2(512) NOT NULL UNIQUE, -- UNIQUE constraint
INSTANCENAME VARCHAR2(64) NOT NULL UNIQUE,
HOSTNAME VARCHAR2(32) NOT NULL,
CONSTRAINT PK_INSTANCES PRIMARY KEY (INSTANCEID, HOSTNAME)
);
Edit: Ok, after reading your comment, try this:
SELECT con.constraint_name, col.column_name, con.constraint_type
FROM user_cons_columns col
JOIN user_constraints con ON (col.constraint_name = con.constraint_name)
WHERE col.table_name = 'INSTANCES'
AND con.constraint_type = 'U'
;
It will list UNIQUE constraints and associated columns for INSTANCE table. Please check if there is a unique constraint on the INSTANCEID column (and if that constraint has no other associated columns).
Example at SQLFiddle: http://sqlfiddle.com/#!4/43b43/6
Edit #2: creating named constraints, all options:
-- CREATE TABLE - "In Line" Constraints
CREATE TABLE ports (
ID NUMBER CONSTRAINT PORT_ID_PK PRIMARY KEY,
NAME VARCHAR2(20)
);
CREATE TABLE ports (
ID NUMBER,
NAME VARCHAR2(20) CONSTRAINT NAME_NN NOT NULL
);
CREATE TABLE ports (
ID NUMBER,
NAME VARCHAR2(20) CONSTRAINT NAME_UQ UNIQUE
);
CREATE TABLE ports (
ID NUMBER,
STATUS NUMBER CONSTRAINT PROPER_STATUS_CK
CHECK (STATUS IN (4, 5))
);
CREATE TABLE ships (
SHIP_ID NUMBER,
NAME VARCHAR2(20),
HOME_PORT_ID NUMBER CONSTRAINT SHIP_PORT_FK
REFERENCES PORTS (ID)
);
-- CREATE TABLE - "Out of Line" Constraints
CREATE TABLE ports (
ID NUMBER,
NAME VARCHAR2(20),
CONSTRAINT PORT_ID_PK PRIMARY KEY (ID)
);
-- NOT NULL constraints can not be created "Out of Line"!
CREATE TABLE ports (
ID NUMBER,
NAME VARCHAR2(20),
CONSTRAINT NAME_UQ UNIQUE (NAME)
);
CREATE TABLE ports (
ID NUMBER,
STATUS NUMBER,
CONSTRAINT PROPER_STATUS_CK
CHECK (STATUS IN (4, 5))
);
CREATE TABLE ships (
SHIP_ID NUMBER,
NAME VARCHAR2(20),
HOME_PORT_ID NUMBER,
CONSTRAINT SHIP_PORT_FK FOREIGN KEY
(HOME_PORT_ID) REFERENCES PORTS (ID)
);
-- ALTER TABLE - "In Line" Constraints
ALTER TABLE PORTS MODIFY ID
CONSTRAINT PORT_ID_PK PRIMARY KEY;
ALTER TABLE PORTS MODIFY NAME
CONSTRAINT NAME_NN NOT NULL;
ALTER TABLE PORTS MODIFY NAME
CONSTRAINT NAME_UQ UNIQUE;
ALTER TABLE SHIPS MODIFY HOME_PORT_ID
CONSTRAINT SHIP_PORT_FK REFERENCES PORTS (ID);
-- ALTER TABLE - "Out of Line" Constraints
ALTER TABLE PORTS ADD CONSTRAINT
PORT_ID_PK PRIMARY KEY (ID);
-- NOT NULL constraints can not be created "Out of Line"!
ALTER TABLE PORTS ADD CONSTRAINT
NAME_UQ UNIQUE (NAME);
ALTER TABLE PORTS ADD
CONSTRAINT PROPER_STATUS_CK
CHECK (STATUS IN (4, 5));
ALTER TABLE SHIPS ADD CONSTRAINT SHIP_PORT_FK
FOREIGN KEY (HOME_PORT_ID)
REFERENCES PORTS (ID);
NOT NULL constraints cannot be create of out line.
I have a composite primary key in my Candidate table
CREATE TABLE CANDIDATE(
CANDIDATE_ID VARCHAR(5),
NAME VARCHAR(30),
TELEPHONE NUMBER,
PRIMARY KEY(CANDIDATE_ID, NAME));
When I create a child table, I get an error saying the number of referencing columns must match referenced columns when I create a foreign key for the CANDIDATE_ID
CREATE TABLE JOB(
POSITION_ID VARCHAR(5) PRIMARY KEY,
CANDIDATE_ID VARCHAR(5),
DATE2 DATE,
FOREIGN KEY(CANDIDATE_ID) REFERENCES CANDIDATE);
A table can only have one primary key-- you have a composite primary key. If you have a composite primary key, you have to reference the entire key in your child table. That would mean that the child table would need to have a CANDIDATE_ID column and a NAME column.
CREATE TABLE job (
position_id VARCHAR2(5) PRIMARY KEY,
candidate_id VARCHAR2(5),
name VARCHAR2(30),
date2 DATE,
FOREIGN KEY( candidate_id, name ) REFERENCES candidate( candidate_id, name )
);
Of course, you probably don't want to store the name in both tables. You probably want the candidate_id to be the prmiary key of candidate and you may want to create a separate unique constraint on name.
CREATE TABLE CANDIDATE(
CANDIDATE_ID VARCHAR(5) primary key,
NAME VARCHAR(30) unique,
TELEPHONE NUMBER);
CREATE TABLE JOB(
POSITION_ID VARCHAR(5) PRIMARY KEY,
CANDIDATE_ID VARCHAR(5),
DATE2 DATE,
FOREIGN KEY(CANDIDATE_ID) REFERENCES CANDIDATE(candidate_id));
Assuming that the combination of CANDIDATE_ID and NAME is required for the key to be unique, then you will need to add a reference to the NAME column in your referencing table.
I suspect that CANDIDATE_ID is enough to uniquely identify the candidates in your primary table. If that is the case then it should be your primary key and your relationship will work. If you want to index the NAME separately then do so, but leave it out of the primary key.
Last line should be like this;
CONSTRAINT FK_CANDIDATE_ID FOREIGN KEY (CANDIDATE_ID)REFERENCES CANDIDATE(CANDIDATE_ID);
CREATE TABLE dept
( did char(3) not null,
dname varchar2(20) not null,
CONSTRAINT dept_pk PRIMARY KEY (did)
);
strong text
create table emp
(
eid char(3) unique,
ename varchar2(10) not null,
sal number check (sal between 20000 AND 50000),
city varchar2(10) default 'texus',
did char(3) not null,
constraint fk_did_dept
FOREIGN KEY (did) references
dept(did)
);