- UNIQUE constraint same as primary key different is UNIQUE allow single NULL value.
- UNIQUE constraint, every time you insert a new row, PostgreSQL checks if the value is already in the table. If it found that the new value is already there, it would give back an error message and reject the changes. The same process is carried out for the update existing data.
- When you add a UNIQUE constraint to a column or a group of columns, PostgreSQL will create a btree index on the respective column or a group of columns automatically.
- The Value of the column must be unique across the whole table. However, the column can have many NULL values because PostgreSQL treats each NULL value to be unique. Notice that SQL standard only allows one NULL value in the column that has the UNIQUE constraint.
CREATING UNIQUE CONSTRAINT FOLLOWING ANYONE METHOD :
1. COLUMN METHOD UNIQUE CONSTRAINT
CREATE TABLE table_name (
Column_1 integer UNIQUE,
Column_2 text,
Column_3 numeric
);
2.TABLE METHOD UNIQUE CONSTRAIN:
CREATE TABLE table_name (
Column_1 integer,
Column_2 text,
Column_3 numeric,
UNIQUE (column_1)
);
3.ALTER METHOD UNIQUE CONSTRAINT:
ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column_1,column_2,.,.,....etc);
PRACTICAL 1. CREATION OF UNIQUE CONSRAINTS:
The following CREATE TABLE statement creates a new table named student with a UNIQUE constraint applied to the student_id column.
postgres=# CREATE TABLE STUDENTS (
student_id integer UNIQUE,
name text,
class integer
);
postgres=# \d students
Table "public.students"
Column | Type | Modifiers
------------+---------+-----------
student_id | integer |
name | text |
class | integer |
Indexes:
"students_student_id_key" UNIQUE CONSTRAINT, btree (student_id)
Note:“students_student_id_key” are the system generated unique key.
PRACTICAL 2. CREATION OF UNIQUE CONSRAINTS WITH CONSTRAINT NAME:
The UNIQUE constraint can be written as the table constraint with constraint name as following example.
postgres=# CREATE TABLE INDIA(
STATE_no integer,
STATE_name character(35),
STATE_grade character(1),
CONSTRAINT uni_key UNIQUE(STATE_no)
);
postgres=# \d india
Table "public.india"
Column | Type | Modifiers
-------------+---------------+-----------
state_no | integer |
state_name | character(35) |
state_grade | character(1) |
Indexes:
"uni_key" UNIQUE CONSTRAINT, btree (state_no)
Note:Here “uni_key” is the unique constraint name.
PRACTICAL 3. INSERTING OPERATION ON UNIQUE KEY TABLE(STUDENTS):
- when you inserting the New value PostgreSQL checks if the value is already in the table. If it found that the new value is already there, it would give back an error message and reject the changes. The same process is carried out for the update existing data.
- the table column can have many NULL values because PostgreSQL treats each NULL value to be unique. Notice that SQL standard only allows one NULL value in the column that has the UNIQUE constraint.
postgres=# insert into students values(1,'nijam',8),(2,'junaith',10),(3,'benz',12);
INSERT 0 3
postgres=# insert into students values(4,'nijam',8),(5,'junaith',10),(null,'benz',12);
INSERT 0 3
postgres=# insert into students values(8,'nijam',8),(9,'junaith',10),(null,'benz',12);
INSERT 0 3
postgres=# insert into students values(11,'nijam',8),(10,'junaith',10),(null,'benz',12);
INSERT 0 3
postgres=# select * from students;
student_id | name | class
------------+---------+-------
1 | nijam | 8
2 | junaith | 10
3 | benz | 12
4 | nijam | 8
5 | junaith | 10
| benz | 12
8 | nijam | 8
9 | junaith | 10
| benz | 12
11 | nijam | 8
10 | junaith | 10
| benz | 12
(12 rows)
when you inserting duplicate values Postgresql will throw error, see following exaple you will understood unique constraints
postgres=# insert into students values(11,'nijam',8),(10,'junaith',10),(12,'benz',12); ERROR: duplicate key value violates unique constraint "students_student_id_key" DETAIL: Key (student_id)=(11) already exists.
–why becouse means value 11 and 12 is already exist in students table
PRACTICAL 4. UNIQUE CONSTRAINT ON MULTIPLE COLUMN:
postgres=# CREATE TABLE batch(
batch_id integer,
batch_name character(35),
members integer,
CONSTRAINT mul_uni_key UNIQUE(batch_id,members)
);
CREATE TABLE
postgres=# \d batch
Table "public.batch"
Column | Type | Modifiers
------------+---------------+-----------
batch_id | integer |
batch_name | character(35) |
members | integer |
Indexes:
"mul_uni_key" UNIQUE CONSTRAINT, btree (batch_id, members)
PRACTICAL 5. INSERTING OPERATION ON COMPOSITE UNIQUE KEY TABLE(BATCH):
postgres=# insert into batch values(1,'nijam',1); INSERT 0 1 postgres=# insert into batch values(2,'nijam',5); INSERT 0 1 postgres=# insert into batch values(3,'nijam',5); INSERT 0 1 postgres=# insert into batch values(4,'nijam',5); INSERT 0 1 postgres=# insert into batch values(4,'nijam',8); INSERT 0 1
postgres=# select * from batch;
batch_id | batch_name | members
----------+-------------------------------------+---------
1 | nijam | 1
2 | nijam | 5
3 | nijam | 5
4 | nijam | 5
4 | nijam | 8
(5 rows)
Note:A table can have at most one UNIQUE (There can be any number of unique and not-null constraints, which are functionally almost the same thing, but only one can be identified as the UNIQUE KEY.)
postgres=# insert into batch values(4,'nijam',8); ERROR: duplicate key value violates unique constraint "mul_uni_key" DETAIL: Key (batch_id, members)=(4, 8) already exists.
Solution:
Postgresql table will have any number of unique and not-null constraints when you inserting the values any one value should be identified as the UNIQUE values other wise it will throw error look above example.
PRACTICAL 6. UNIQUE CONSTRAINT USING UNIQUE INDEX:
If you want to add a unique constraint to a column or a group of columns using existing unique index. Let’s take a look at the following steps.
step 1.Suppose we have a table named cellphones:
create table cellphones( id int, name VARCHAR (50) NOT NULL, phone_model VARCHAR (16) NOT NULL ); CREATE TABLE
step 2. We create a unique index based on the phone_model column.
postgres=# CREATE UNIQUE INDEX CONCURRENTLY uni_index ON cellphones(phone_model); CREATE INDEX
step 3. Add a unique constraint to the cellphones table using the “uni_index” index.
postgres=# ALTER TABLE cellphones ADD CONSTRAINT uni_constraint UNIQUE USING INDEX uni_index;
NOTICE: ALTER TABLE / ADD CONSTRAINT USING INDEX will rename index "uni_index" to "uni_constraint"
ALTER TABLE
postgres=# \d cellphones
Table "public.cellphones"
Column | Type | Modifiers
-------------+-----------------------+---------------------------------------------------------
id | integer | not null default nextval('cellphones_id_seq'::regclass)
name | character varying(50) | not null
phone_model | character varying(16) | not null
Indexes:
"cellphones_pkey" PRIMARY KEY, btree (id)
"uni_constraint" UNIQUE CONSTRAINT, btree (phone_model)
step 4. MONITOR THE PENDING TRANSACTION USING PG_STAT_ACTIVITY:
postgres=# select datid,datname,usename,state_change,state,query_start,waiting from pg_stat_activity; datid | datname | usename | state_change | state | query_start | waiting -------+----------+----------+-------------------------------+--------+-------------------------------+--------- 12814 | postgres | postgres | 2017-04-02 12:54:56.761781-04 | active | 2017-04-02 12:54:56.761768-04 | f 12814 | postgres | postgres | 2017-03-30 11:09:38.073502-04 | idle | 2017-03-30 11:09:38.03711-04 | f (2 rows)
Note: ALTER TABLE statement needs an exclusive lock on the table. If you have many pending transactions, it will wait for those to complete before changing the table. You should check the pg_stat_activity table to see how many pending transactions are in place using the following query:
You should look at the result to find the state column with the value idle in transaction. Those are the transactions that are pending to complete.
PRACTICAL 7. UNIQUE CONSTRAINT ON MULTIPLE COLUMN USING ALTER TABLE:
postgres=# CREATE TABLE LOCATION (
No integer,
village VARCHAR (50) NOT NULL,
area_code integer
);
postgres=# \d LOCATION
Table "public.location"
Column | Type | Modifiers
-----------+-----------------------+-----------
no | integer |
village | character varying(50) | not null
area_code | integer |
–To add a (multicolumn) unique constraint to a table
postgres=# ALTER TABLE LOCATION ADD CONSTRAINT dist_id_zipcode_key UNIQUE (No,area_code);
ALTER TABLE
postgres=# \d LOCATION
Table "public.location"
Column | Type | Modifiers
-----------+-----------------------+-----------
no | integer |
village | character varying(50) | not null
area_code | integer |
Indexes:
"dist_id_zipcode_key" UNIQUE CONSTRAINT, btree (no, area_code)
PRACTICAL 8. DROP UNIQUE KEY USING ALTER TABLE STATEMENT:
postgres=# alter table LOCATION drop constraint dist_id_zipcode_key ;
ALTER TABLE
postgres=# \d LOCATION
Table "public.location"
Column | Type | Modifiers
-----------+-----------------------+-----------
no | integer |
village | character varying(50) | not null
area_code | integer
PRACTICAL 9. POSTGRESQL CONSTRAINT VIEWS:
if you want see all constraint name and table name(relname) you need to join pg_stat_all_tables and pg_constraint views
postgres=# select a.conname,a.contype,a.conrelid,b.relname,b.relid from pg_constraint a,pg_stat_all_tables b where a.conrelid=b.relid;
conname | contype | conrelid | relname | relid
-------------------------+---------+----------+----------------+-------
prikey | p | 24676 | films | 24676
production | u | 24697 | k | 24697
distributorc_did_check | c | 24702 | distributorc | 24702
uni_key | u | 24761 | india | 24761
code_title | p | 24710 | primtab | 24710
students_student_id_key | u | 24753 | students | 24753
pkey | p | 24735 | postgres3 | 24735
uni_constraint2 | u | 24784 | mobiles | 24784
mobiles_pkey | p | 24784 | mobiles | 24784
uni_constraint | u | 24773 | cellphones | 24773
cellphones_pkey | p | 24773 | cellphones | 24773
con1 | c | 24706 | distributorsct | 24706
postgres1_order_no_key | u | 24725 | postgres1 | 24725
mul_uni_key | u | 24766 | batch | 24766
distributors_pkey | p | 24690 | distributors | 24690
distributors_name_check | c | 24690 | distributors | 24690
postgres_ord_no_key | u | 24715 | postgres | 24715
(17 rows)
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