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Author: Unique982

AUTO INCREMENT in SQL.txt

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+// Auto increment 
+-> Auto increment is used to generate an unique number when a new record is inserted into a table.
+ex:create table staff ( s_id int(10) NOT NULL AUTO_INCREMENT,name varcha
+r (50), number int (10),PRIMARY KEY (s_id) );
+Query OK, 0 rows affected, 2 warnings (0.01 sec)
+
+mysql> desc staff;
++--------+-------------+------+-----+---------+----------------+
+| Field  | Type        | Null | Key | Default | Extra          |
++--------+-------------+------+-----+---------+----------------+
+| s_id   | int         | NO   | PRI | NULL    | auto_increment |
+| name   | varchar(50) | YES  |     | NULL    |                |
+| number | int         | YES  |     | NULL    |                |
++--------+-------------+------+-----+---------+----------------+
+
+ex: INSERT INTO staff(name,number) VALUES ('Hari',4567890);
+mysql> select * from staff;
++------+------+---------+
+| s_id | name | number  |
++------+------+---------+
+|    1 | Hari | 4567890 |
++------+------+---------+

NOT NULL in SQL.txt

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+// NOT NULL
+-> By default a tables column can hold NULL values.
+The NOT NULL constraint enforces a field to always contain a vlaue. This means that you cannot insert a new record, or update a record without 
+adding a value to this field.
+ex:create table example(name varchar (30),roll int (10), number int(10)
+NOT NULL);
+
+// output
+desc example;
++--------+-------------+------+-----+---------+-------+
+| Field  | Type        | Null | Key | Default | Extra |
++--------+-------------+------+-----+---------+-------+
+| name   | varchar(30) | YES  |     | NULL    |       |
+| roll   | int         | YES  |     | NULL    |       |
+| number | int         | NO   |     | NULL    |       |
++--------+-------------+------+-----+---------+-------+
+
+msg :  INSERT INTO example (name,roll) VALUES('Ram',10);
+ERROR 1364 (HY000): Field 'number' doesn't have a default value
+mysql>

PRIMARY KEY in SQL.txt

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+// PRIMARY Key
+-> The PRIMARY KEY constraint uniquely identifies each record in a database table.
+Primaray keys must contains UNIQUE values. A primary key column cannot contain NULL values.
+Most tables should have a primary key, and each table can have only ONE primary key.
+syntax: CREATE TABLE example(s_id int (10) NOT NULL PRIMARY KEY ,name varchar
+(50), roll_no int (10) ,number int (10));
+ 
+ ex: desc example;
++---------+-------------+------+-----+---------+-------+
+| Field   | Type        | Null | Key | Default | Extra |
++---------+-------------+------+-----+---------+-------+
+| s_id    | int         | NO   | PRI | NULL    |       |
+| name    | varchar(50) | YES  |     | NULL    |       |
+| roll_no | int         | YES  |     | NULL    |       |
+| number  | int         | YES  |     | NULL    |       |
++---------+-------------+------+-----+---------+-------+

UNIQUE KEY in SQL.txt

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+// UNIQUE Key
+-> The UNIQUE constraint uniquely identifies each record in a database table.
+There can be many UNIQUE constraints per table. A Unique key column can contain NULL values.
+ex: CREATE TABLE example(s_id int (10) UNIQUE ,name varchar(50), roll_no
+int (10) UNIQUE  ,number int (10));
+
+output:desc example;
++---------+-------------+------+-----+---------+-------+
+| Field   | Type        | Null | Key | Default | Extra |
++---------+-------------+------+-----+---------+-------+
+| s_id    | int         | YES  | UNI | NULL    |       |
+| name    | varchar(50) | YES  |     | NULL    |       |
+| roll_no | int         | YES  | UNI | NULL    |       |
+| number  | int         | YES  |     | NULL    |       |
++---------+-------------+------+-----+---------+-------+
+
+ex: insert into example (s_id,name,number) VALUES(2,'RAM',9888888);
+mysql> select * from example;
++------+------+---------+---------+
+| s_id | name | roll_no | number  |
++------+------+---------+---------+
+|    1 | RAM  |       1 | 9888888 |
+|    2 | RAM  |    NULL | 9888888 |
++------+------+---------+---------+