One query that is one often encounters is to answer the question, given data in a database table, how do we obtain the TOP N based on some column of the table ?
There are a few ways to do this, but I will present the most popular 2 methods.
I will illustrate using the following table, emp:
SELECT * FROM emp;
+-------+--------+-----------+------+------------+---------+---------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+-------+--------+-----------+------+------------+---------+---------+--------+
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 |
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 |
| 7788 | SCOTT | ANALYST | 7566 | 1982-12-09 | 3000.00 | NULL | 20 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 |
| 7876 | ADAMS | CLERK | 7788 | 1983-01-12 | 1100.00 | NULL | 20 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 |
+-------+--------+-----------+------+------------+---------+---------+--------+
14 rows in set (0.00 sec)
Suppose we wish to obtain the top 5 employees by salary, how can one do this ?
- Using the RANK() function.
In databases which provide the RANK function, we can obtain the top 5 employees
by salary using the following query:SELECT empno, ename, sal
FROM (SELECT empno, ename, sal, RANK() OVER (ORDER BY sal DESC) sal_rank
FROM emp)
WHERE sal_rank <= 5;+-------+-------+---------+
| empno | ename | sal |
+-------+-------+---------+
| 7839 | KING | 5000.00 |
| 7788 | SCOTT | 3000.00 |
| 7902 | FORD | 3000.00 |
| 7566 | JONES | 2975.00 |
| 7698 | BLAKE | 2850.00 |
+-------+-------+---------+
5 rows returned in 0.02 seconds - Using an expression that limits the number of rows returned from an ordered SQL result set.
The list of expressions for the databases are shown below:
| Database | Expression |
|---|---|
| Oracle | ROWCOUNT |
| MySQL/PostgreSQL/Vertica | LIMIT |
| Sybase | ROWNUM |
| MS SQL | TOP |
ii. Here are the corr. queries for each database:
Vertica/PostgreSQL/MySQL
SELECT empno, ename, sal FROM (SELECT empno, ename, sal ORDER BY sal DESC) a
LIMIT 5;
Oracle
SELECT * FROM (SELECT empno, ename, sal FROM emp ORDER BY sal DESC) A WHERE ROWNUM <= 5;
Sybase
SET ROWCOUNT 10;
SELECT empno, ename, sal FROM emp ORDER BY sal DESC;
MS SQL
SELECT TOP 5 FROM (SELECT empno, ename, sal FROM emp ORDER BY sal DESC);