There are 5 types
1.data definition type
SQL is Structured Query Language is a database computer language designed for managing data in relational database management systems (RDBMS).
PostgreSQL is an object-relational database management system (ORDBMS).It is released under a BSD-style license and is thus free software. As with many other open-source programs, PostgreSQL is not controlled by any single company, but has a global community of developers and companies to develop it.
SQLite is an ACID-compliant embedded relational database management system contained in a relatively small (~225 KB) C programming library. The source code for SQLite is in the public domain.
MySQL is a relational database management system (RDBMS) which has more than 6 million installations. MySQL stands for "My Structured Query Language". The program runs as a server providing multi-user access to a number of databases.
A Cartesian product is the result of an unrestricted join of two or more tables. The result set of a three table Cartesian product will have x * y * z number of rows where x, y, z correspond to the number of rows in each table involved in the join.
Integrity constraint is a rule that restricts values to a column in a table.
Use the DISTINCT keyword right after SELECT...
i.e. SELECT DISTINCT customername FROM customer
select * from emp e where rownum=
(select max(rownum) from emp ee