SQL UNIQUE uniquely identifies each record in a database table.

The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.

A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.

We can have many UNIQUE constraints in a table, but only one PRIMARY KEY .

Example

mysql> CREATE TABLE  `new_users` (`id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(255) NOT NULL, PRIMARY KEY (`id`), UNIQUE (Id));
Query OK, 0 rows affected (0.10 sec)

SQL NOT NULL  make sure that a column does not accept null values

Example

mysql> CREATE TABLE  `new_users` (`id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(255) NOT NULL, PRIMARY KEY (`id`));
Query OK, 0 rows affected (0.09 sec)

Here the field will never have a null value

Constraints are used to limit the type of data that can go into a table and it can be specified when a table is created or after the table is created.

Below given are some important constraints

PRIMARY KEY
FOREIGN KEY
NOT NULL
UNIQUE
CHECK
DEFAULT

SQL CREATE TABLE is used to create a table.

Syntax

CREATE TABLE table_name

Example

mysql> CREATE TABLE `new_users` (`id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(255) NOT NULL, PRIMARY KEY (`id`));

Query OK, 0 rows affected (0.09 sec)

SQL CREATE DATABASE is used to create a database.

Syntax

CREATE DATABASE database_name

Example

CREATE DATABASE phpcode;

SQL SELECT INTO statement selects data from one table and inserts it into a different table

Mainly it can be used to keep the backup of the table .

Syntax

SELECT *  INTO new_table_name [IN another_DB] FROM old_tablename

SQL UNION can be  used to combine the result  two or more SELECT statements.

Notice that each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order.

Syntax

SELECT column_name(s) FROM table_name1
UNION
SELECT column_name(s) FROM table_name2

MySQL Table – user1

+—-+——+
| id | name |
+—-+——+
|  1 | AAAA |
|  2 | BBBB |
|  3 | CCCC |
|  4 | DDDD |
+—-+——+

MySQL Table – user2

+—-+——+
| id | name |
+—-+——+
|  1 | PPPP |
|  2 | QQQQ |
|  3 | RRRR |
|  4 | RRRR |
+—-+——+

Example

mysql> SELECT * FROM user1 UNION SELECT * FROM user2 ;
+—-+——+
| id | name |
+—-+——+
|  1 | AAAA |
|  2 | BBBB |
|  3 | CCCC |
|  4 | DDDD |
|  1 | PPPP |
|  2 | QQQQ |
|  3 | RRRR |
|  4 | RRRR |
+—-+——+
8 rows in set (0.00 sec)

SQL RIGHT JOIN  returns all the rows from the right table even if there are no matches in the left table

Syntax

SELECT * FROM table_name1 RIGHT JOIN table_name2
ON table_name1.column_name=table_name2.column_name

MySQL Table – users

|+—-+——+
| id | name |
+—-+——+
|  1 | AAAA |
|  2 | BBBB |
|  3 | CCCC |
|  4 | DDDD |
|  5 | EEEE |
+—-+——+

MySQL Table – Orders

+—–+—–+——–+
| oid | uid | items  |
+—–+—–+——–+
|   1 |   1 | Pen    |
|   2 |   2 | Watch  |
|   3 |   3 | shoe   |
|   4 |   4 | mobile |
+—–+—–+——–+

Example

mysql> SELECT u.name,o.items FROM users as u RIGHT JOIN orders as o ON u.id=o.uid ;
+——+——–+
| name | items  |
+——+——–+
| AAAA | Pen    |
| BBBB | Watch  |
| CCCC | shoe   |
| DDDD | mobile |
+——+——–+
4 rows in set (0.00 sec)

SQL LEFT JOIN  returns all rows from the left table even if there are no matches in the right table.

Syntax

SELECT * FROM table_name1 LEFT JOIN table_name2
ON table_name1.column_name=table_name2.column_name

MySQL Table – users

|+—-+——+
| id | name |
+—-+——+
|  1 | AAAA |
|  2 | BBBB |
|  3 | CCCC |
|  4 | DDDD |
|  5 | EEEE |
+—-+——+

MySQL Table – Orders

+—–+—–+——–+
| oid | uid | items  |
+—–+—–+——–+
|   1 |   1 | Pen    |
|   2 |   2 | Watch  |
|   3 |   3 | shoe   |
|   4 |   4 | mobile |
+—–+—–+——–+

Example

mysql> SELECT u.name,o.items FROM users as u LEFT JOIN orders as o ON u.id=o.uid ;
+——+——–+
| name | items  |
+——+——–+
| AAAA | Pen    |
| BBBB | Watch  |
| CCCC | shoe   |
| DDDD | mobile |
| EEEE | NULL   |
+——+——–+
5 rows in set (0.01 sec)