Oracle MAX function

The Oracle/PLSQL MAX function returns the maximum value of the expression.

Oracle/PLSQL syntax of the MAX function

SELECT MAX(aggregate_expression_id)
FROM tabs
[WHERE conds]

OR syntax for MAX function with results grouped in one or more columns:

SELECT expression1_id, expression2_id, ... expression_n_id,
MAX(aggregate_expression_id)
FROM tabs
[WHERE conds]
GROUP BY expression1_id, expression2_id, ... expression_n_id;

Parameters and arguments of the function

• expression1_id, expression2_id, … expression_n_id – expressions that are not encapsulated in the MAX function and must be included in the GROUP BY statement at the end of the SQL sentence.
• aggregate_expression_id – is a column or expression from which the maximum value will be returned.
• tabs – tables from which you want to get records. At least one table must be specified in FROM operator.
• WHERE conds – optional. These are the conditions that must be met for the selected records.

The MAX function can be used in the following versions of Oracle/PLSQL

Example with a single field

Let’s consider some examples of the MAX function and learn how to use the MAX function in Oracle/PLSQL.
For example, you may want to know what the maximum salary of all employees is.

SELECT MAX(salary_id) AS "Highest Salary" FROM empls;

In this example of the MAX function, the MAX(salary) expression is represented under the “Highest Salary” alias. As a result, “Highest Salary” will be displayed as the field name when the resulting set returns.

Example – using GROUP BY

In some cases, you will need to use GROUP BY grouping with the MAX function.

For example, you could use the MAX function to return the department name and MAX(salary).

SELECT depart, MAX(salary_id) AS "Highest salary".
FROM empls
GROUP BY depart;

Since your SELECT operator has one column that is not encapsulated in the MAX function, you must use the GROUP BY operator. Therefore, the field department must be specified in the operator GROUP BY.

FAQ

Q: I’m trying to get some information out of the table. For simplicity, say the table (report_history) has 4 columns: user_name, report_job_id, report_name and report_run_date.

Each time a report is executed in Oracle, the record is fixed in the table, marking the above information. What I am trying to do is to pull out of this table when and by whom the last single report was run.

My initial request:

SELECT report_name, MAX(report_run_date)
FROM report_history
GROUP BY report_name

works fine. However, this does not give the name of the user who started the report.

Adding user_name to the sample list and to the GROUP BY operator returns a few lines for each report; the results show the last time each person started each report is a question. (i.e. User1 started report1 01-July-14, User2 started report1 01-AUG-14). I don’t want to do this … I just want to know who launched a particular report and when it was last launched.

Any suggestions?

Answer: This is all getting a little more complicated. In the query below, SQL SELECT will return the results you want:

SELECT rh.user_name,
rh.report_name,
rh.report_run_date
FROM report_history rh,
(SELECT MAX(report_run_date) AS maxdate,
report_name
FROM report_history
GROUP BY report_name) maxresults
WHERE rh.report_name = maxresults.report_name
AND rh.report_run_date= maxresults.maxdate;

First, we will assign an alias rh to the first instance of the report_history table.
Second, we have included two components into the FROM operator. The first of these is the report_history table (alias RH). The second is a subquery with the nickname maxresults:

(SELECT MAX(report_run_date) AS maxdate, report_name
FROM report_history
GROUP BY report_name) maxresults

We have the alias MAX(report_run_date) as maxdate and the alias of the resulting set as maxresults.

Now that we have compiled this query within our FROM, Oracle will allow us to combine these results from our source report_history table. In this way, we have combined the report_name and report_run_date fields of the table with the aliases rh and maxresults. This allows us to get report_name, MAX(report_run_date), and user_name.

Using the MAX and MIN functions (Introduction to Oracle SQL)