I created a before delete trigger:
create or replace trigger myTrigger3
before delete on emp
for each row
begin
update emp set mgr = 'Null' where mgr = :old.emp_name;
end;
Where table is
emp(emp_id integer primary key, emp_name varchar(20), mgr varchar(20))
But when I run this statement the trigger is not running.
delete from emp where emp_id = 1004;
select * from emp;
Error report -
ORA-04091: table DB20178004.EMP is mutating, trigger/function may not see it
ORA-06512: at "DB20178004.MYTRIGGER3", line 2
ORA-04088: error during execution of trigger 'DB20178004.MYTRIGGER3'
You can prefer adding a foreign key constraint with set null option instead of such a trigger. Of course you need a primary key should already been defined on emp_id column :
alter table emp
add constraint fk_mgr foreign key(mgr)
references emp(emp_id)
on delete set null;
Whenever you delete the record with an emp_id which has matching values with mgr column those will be emptied after deletion of the record with that emp_id.
But please prefer a data type(numeric) for mgr conforming with the column
emp_id such as integer to be able to define a foreign key
constraint.
Demo
By the way,
I recommend you to use soft-deletion. e.g. adding a column active to the table and
set value of it to zero whenever want to delete, and do not show the
records with active=0 on the application.
If you insisting on deletion do not filter by emp_name column, since
there can be more than one people with the common name, but using emp_id
is better by far as being unique within the table.
I have tables with emp1 and emp2
emp1:
emp_1 | emp_2
1 | 2
3 | 4
5 | 6
emp2:
emp
1
2
3
6
I tried to set primary key to table emp1 and foreign key to emp2.
My code:
For primary key:
alter table emp1 add primary key(emp_1,emp_2);
For foreign key:
alter table emp2
add foreign key (emp)
references a_t1(emp_1,emp_2);
Error:
Error report -
SQL Error: ORA-02256: number of referencing columns must match referenced columns
02256. 00000 - "number of referencing columns must match referenced columns"
*Cause: The number of columns in the foreign-key referencing list is not
equal to the number of columns in the referenced list.
*Action: Make sure that the referencing columns match the referenced
columns.
Kindly help me to solve this error and set the primary key.
The only way I can think to do it is a nasty hack involving a materialized view. It would be better to fix your data so that you don't have the primary key spread across two columns.
CREATE TABLE EMP1 (
EMP_1 INT UNIQUE,
EMP_2 INT UNIQUE,
PRIMARY KEY ( EMP_1,EMP_2 )
);
CREATE MATERIALIZED VIEW LOG ON EMP1
WITH SEQUENCE, ROWID(EMP_1, EMP_2)
INCLUDING NEW VALUES;
CREATE MATERIALIZED VIEW EMP1_MV
BUILD IMMEDIATE
REFRESH FAST ON COMMIT
AS SELECT EMP_1 AS EMP
FROM EMP1
UNION ALL
SELECT EMP_2
FROM EMP1;
ALTER TABLE EMP1_MV ADD CONSTRAINT EMP1_MV__PK PRIMARY KEY ( EMP );
CREATE TABLE EMP2 (
EMP INT PRIMARY KEY REFERENCES EMP1_MV( EMP )
);
I have 3 tables in my database namely employees,students and Images
create table employees(id number primary key,name varchar2(100), address varchar2(100));
create table students(id number primary key,name varchar2(100),address varchar2(100));
create table Images (image_id number primary key,employee_id number,student_id number,image_name varchar2(100));
Insert into employees values (1,'asdfasd','asdfasdf');
Insert into employees values (2,'asdfasd','asdfasdf');
Insert into employees values (3,'asdfasd','asdfasdf');
Insert into employees values (4,'asdfasd','asdfasdf');
Insert into employees values (5,'asdfasd','asdfasdf');
Insert into students values (1,'asdfasd','asdfasdf');
Insert into students values (2,'asdfasd','asdfasdf');
Insert into students values (3,'asdfasd','asdfasdf');
Insert into students values (4,'asdfasd','asdfasdf');
Insert into students values (5,'asdfasd','asdfasdf');
Insert into students values (49,'asdfasd','asdfasdf');
Insert into Images(image_id,employee_id,image_name) values (1,5,'adsfasdfasdf');
Insert into Images(image_id,student_id,image_name) values (2,49,'asfasdfasdf');
Now, when Inserting a row into the Images table I should check whether the employee_id / Student_id is existed in the employees table/student table, If a match found then only I have to Insert else it should not.
I thought of adding two foreign keys on the images table as follows:
alter table images add constraint fk_employeeid foreign key(employee_id)
references employees(id);
alter table images add constraint fk_studentsid foreign key(student_id)
references students(id);
But, If I do so. It will not allow me to insert null values. How can I modify my design so that whenever I insert a row in images table either it should be an employee_id or an student_id.
If I created any confusion, here is the link for the sql fiddle,http://www.sqlfiddle.com/#!4/92d24/1/0
I thought of adding two foreign keys on the images table.
But, If I do so. It will not allow me to insert null values.
You are wrong when you say the foreign key constraint won't allow NULL values. It will definitely allow the NULL values.
Test case
Let's add the foreign key constraint on the IMAGES table for the employee_id and student_id.
ALTER TABLE images ADD CONSTRAINT fk_emp FOREIGN KEY(employee_id) REFERENCES employees(ID);
ALTER TABLE images ADD CONSTRAINT fk_stu FOREIGN KEY(student_id) REFERENCES students(ID);
Let's check the INSERT with NULL values:
SQL> INSERT INTO Images(image_id,employee_id,image_name) VALUES (1,5,'adsfasdfasdf');
1 row created.
SQL> INSERT INTO Images(image_id,student_id,image_name) VALUES (2,49,'asfasdfasdf');
1 row created.
SQL> INSERT INTO Images(image_id,employee_id,image_name) VALUES (3,null,'adsfasdfasdf');
1 row created.
SQL> INSERT INTO Images(image_id,student_id,image_name) VALUES (4,null,'asfasdfasdf');
1 row created.
SQL>
SQL> COMMIT;
Commit complete.
SQL>
SQL> SELECT * FROM images;
IMAGE_ID EMPLOYEE_ID STUDENT_ID IMAGE_NAME
---------- ----------- ---------- ---------------
1 5 adsfasdfasdf
2 49 asfasdfasdf
3 adsfasdfasdf
4 asfasdfasdf
SQL>
Let's check the foreign key constraint validation:
SQL> INSERT INTO Images(image_id,employee_id,image_name) VALUES (1,10,'adsfasdfasdf');
INSERT INTO Images(image_id,employee_id,image_name) VALUES (1,10,'adsfasdfasdf')
*
ERROR at line 1:
ORA-00001: unique constraint (LALIT.SYS_C0010739) violated
SQL> INSERT INTO Images(image_id,student_id,image_name) VALUES (2,20,'asfasdfasdf');
INSERT INTO Images(image_id,student_id,image_name) VALUES (2,20,'asfasdfasdf')
*
ERROR at line 1:
ORA-00001: unique constraint (LALIT.SYS_C0010739) violated
SQL>
So, everything works as per the design.
Add constraints to images like you did. Add one more constraint:
alter table images add constraint chk_nulls
check (
(employee_id is not null and student_id is null)
or (employee_id is null and student_id is not null)
);
This way you cannot insert both nulls nor both not nulls for employee_id and student_id. And foreign keys are also checked.
Test:
insert into images values (1, 1, null, 'not important'); -- OK
insert into images values (2, null, null, 'not important'); -- error
insert into images values (3, 1, 1, 'not important'); -- error
insert into images values (4, null, 1, 'not important'); -- OK
I would keep the foreign keys, that is the correct approach.
But instead of inserting NULL's, you should define an "unknown record" in your EMPLOYEES and STUDENTS tables (maybe with an id=-1, name='UNKNOWN'), and insert -1 into your IMAGES table.
I recommend you to change the primary key in table images, and have as PK three columns (image_id, employee_id and student_id).
Edit 1: Based on comment
I think you planned in a wrong way the problem. If the images are for both, employees and students, you should have two tables, images_employees and images_students, with respectively foreign keys employee_id and student_id and of course an id for the image.
But is there any sense to have a row like this?
id: 1 student_id: null image_name: 'something'
I don't think so... please explain more about the purpose of images table.
Is there some equivalent to the Mysql specific instruction that disable the check of the foreign keys constraints ?
SET FOREIGN_KEY_CHECKS = 0;
There is no command in Oracle that will disable all constraints at once.
However it seems you want to disable constraints in the context of dropping tables. In that case you can use the CASCADE CONSTRAINTS clause to drop the referencing constraints from other tables along with the table being dropped.
Here's an example:
SQL> CREATE TABLE t1 (ID NUMBER PRIMARY KEY);
Table created
SQL> CREATE TABLE t2 (ID NUMBER REFERENCES t1);
Table created
SQL> INSERT INTO t1 VALUES (1);
1 row inserted
SQL> INSERT INTO t2 VALUES (1);
1 row inserted
SQL> -- this fails because of the foreign key
SQL> DROP TABLE t1;
ORA-02449: unique/primary keys in table referenced by foreign keys
SQL> DROP TABLE t1 CASCADE CONSTRAINTS;
Table dropped
SET FOREIGN_KEY_CHECKS = 0; is session based. In an Oracle context I can only imagine you needing to do this when you have circular references.
You've commented that this is what you want to do:
SET FOREIGN_KEY_CHECKS = 0;
DROP TABLE table1;
DROP TABLE table2;
SET FOREIGN_KEY_CHECKS = 1;
I'm assuming that this means that TABLE1 has a foreign key referencing TABLE2 and TABLE2 has a foreign key referencing TABLE1.
If this is the case then Moudiz's answer is correct. You want to disable the foreign keys before dropping the table:
alter table table1 disable constraint <constraint_name>;
alter table table2 disable constraint <constraint_name>;
drop table table1;
drop table table2;
There's no point disabling all foreign keys for the length of the session, you're only interested in two of them, both of which will be dropped with the table.
You don't want to ever disable all foreign keys.
The only other context I can think of this in is if you want to insert something into your circular reference, in which case you would declare the constraint as DEFERRABLE. This means that the constraint check is done at the end of the transaction rather than on DML being performed, see the documentation.
If your references aren't circular then simply drop the tables in the other order.
IF Your asking for a query to disable a foreign key then try this :
ALTER TABLE mytable
disable CONSTRAINT fk_mytable;
Lets say i have a table with the following definition
create table dummy (col1 number(9) not null)
All the values in this dummy.col1 are 7 digit long. Now i want to reduce the length of this column from 9 - 7 using alter command. Oracle gives me error that column to be modified must be empty to decrease precision or scale. Makes sense.
I want to ask is there any work around to reduce the column size?
I can't delete the values in the column.
I can't copy values from this column to another since it has trillions of data.
The column size has no relationship to how the data is physically stored (they are variable length)
e.g. '23' in a number(2) will take exactly the same space if stored in a number(38)
It is purely a constraint on the maximum number that can be stored in the column therefore you could just add a constraint on the column:
ALTER TABLE dummy ADD
CONSTRAINT c1
CHECK (col1 < 9999999)
ENABLE
VALIDATE;
if you want it to go a little quicker change VALIDATE to NOVALIDATE obviously this will not check the validity of the existing data.
Kevin's answer is excellent.
The only other way to do it is to
rename the existing column,
create a new column with the old name and the new size,
issue an update statement to populate the new field (which you said you cannot do)
and then drop the renamed column.
Are you sure you cannot find some downtime one weekend to perform this task ?
Solution #1
My solution below keeps the original column order.
I found that to be important, especially if there are canned SQL statements out
there (middle tier, client tier) that point back to your database that do implicit
SELECTs.
i.e.
SELECT *
FROM tableName
WHERE ...;
INSERT INTO copyTableName(column1,column2,column3,...)
SELECT *
FROM tableName
WHERE ...;
Here goes:
Generate the DDLs for
1. The table containing the column you intend to resize
2. All the relationship constraints, indexes, check constraints, triggers that reference that table.
3. All the foreign keys of other tables that reference the primary key of this table.
Make sure each table-referencing-object DDL is stand-alone, separate from the
CREATE TABLE DDL.
You'll have something like
/* 1. The table containing the column you intend to resize */
CREATE TABLE tableName
(
column1 TYPE(size) [DEFAULT value] [NOT] NULL,
column2 TYPE(size) [DEFAULT value] [NOT] NULL,
column3 TYPE(size) [DEFAULT value] [NOT] NULL,
...
)
TABLESPACE tsName
[OPTIONS];
/* 2. All the relationship constraints, indexes, check constraints, triggers that reference that table. */
CREATE INDEX indexName ON tableName
(column1)
NOLOGGING
TABLESPACE INDX
NOPARALLEL;
CREATE INDEX compositeIndexName ON tableName
(column1,column2,...)
NOLOGGING
TABLESPACE INDX
NOPARALLEL;
CREATE UNIQUE INDEX pkName ON tableName
(column2)
NOLOGGING
TABLESPACE INDX
NOPARALLEL;
ALTER TABLE tableName ADD (
CHECK (column4 IS NOT NULL));
ALTER TABLE tableName ADD (
CONSTRAINT pkName
PRIMARY KEY
(column2)
USING INDEX
TABLESPACE INDX);
ALTER TABLE tableName ADD (
CONSTRAINT fkName
FOREIGN KEY (column2)
REFERENCES otherTable (column2));
/* 3. All the foreign keys of other tables that reference the primary key of this table. */
ALTER TABLE otherTableName ADD (
CONSTRAINT otherTableFkName
FOREIGN KEY (otherTableColumn2)
REFERENCES tableName (column1));
Copy out just the CREATE TABLE statement, change the table name and
reduce the size of the column you wish to modify:
CREATE TABLE tableName_YYYYMMDD
(
column1 TYPE(size) [DEFAULT value] [NOT] NULL,
column2 TYPE(reducedSize) [DEFAULT value] [NOT] NULL,
column3 TYPE(size) [DEFAULT value] [NOT] NULL,
...
)
TABLESPACE tsName
[OPTIONS];
Insert the data from tableName into tableName_YYYYMMDD:
INSERT /* APPEND */ INTO tableName_YYYYMMDD(
column1 ,
column2 ,
column3 ,
... )
SELECT
column1 ,
column2 ,
column3 ,
...
FROM tableName;
COMMIT;
Drop all objects referencing the original table.
Also, drop all foreign keys that reference the tableName primary key pkName.
Don't worry, you've saved the DDL so you'll be able to recreate them.
Notice that I drop indexes after copying the data out of tableName.
I do this because perhaps one of the indexes will be used in the
above SELECT so that operation will complete faster.
DROP INDEX indexName ;
DROP INDEX compositeIndexName ;
DROP UNIQUE INDEX pkName ;
ALTER TABLE tableName DROP CONSTRAINT pkName ;
ALTER TABLE tableName DROP CONSTRAINT fkName ;
ALTER TABLE otherTableName DROP CONSTRAINT otherTableFkName ;
Drop the original table.
DROP TABLE tableName;
Rename the new table.
ALTER TABLE tableName_YYYYMMDD RENAME TO tableName;
Recreate all referencing objects from the DDL statements you saved before.
/* 2. All the relationship constraints, indexes, check constraints, triggers that reference that table. */
CREATE INDEX indexName ON tableName
(column1)
NOLOGGING
TABLESPACE INDX
NOPARALLEL;
CREATE INDEX compositeIndexName ON tableName
(column1,column2,...)
NOLOGGING
TABLESPACE INDX
NOPARALLEL;
CREATE UNIQUE INDEX pkName ON tableName
(column2)
NOLOGGING
TABLESPACE INDX
NOPARALLEL;
ALTER TABLE tableName ADD (
CHECK (column4 IS NOT NULL));
ALTER TABLE tableName ADD (
CONSTRAINT pkName
PRIMARY KEY
(column2)
USING INDEX
TABLESPACE INDX);
ALTER TABLE tableName ADD (
CONSTRAINT fkName
FOREIGN KEY (column2)
REFERENCES otherTable (column2));
/* 3. All the foreign keys of other tables that reference the primary key of this table. */
ALTER TABLE otherTableName ADD (
CONSTRAINT otherTableFkName
FOREIGN KEY (otherTableColumn2)
REFERENCES tableName (column1));
Solution #2
Keep the column order but do not rebuild non-unique-used-by-PK indexes that might contain column2.
ALTER TABLE tableName ADD (column2Copy TYPE(reducedSize));
UPDATE tableName SET column2Copy = column2;
ALTER TABLE tableName MODIFY (column2 TYPE(size) NULL);
ALTER TABLE tableName DROP CONSTRAINT pkName;
DROP INDEX pkName;
UPDATE tableName SET column2 = null;
ALTER TABLE tableName MODIFY (column2 TYPE(reducedSize));
UPDATE tableName SET column2 = column2Copy;
ALTER TABLE tableName DROP COLUMN column2Copy;
CREATE UNIQUE INDEX pkName ON tableName
(column2)
NOLOGGING
TABLESPACE INDX
NOPARALLEL;
ALTER TABLE tableName ADD (
CONSTRAINT pkName
PRIMARY KEY
(column2)
USING INDEX
TABLESPACE INDX);
COMMIT;