Sorting rows using ORDER BY clause

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1. Basic usage of ORDER BY clause
2. The sort expression
3. Using multiple columns to sort
4. Choose Null value show at first or last?

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Basic usage of ORDER BY clause

After a query has produced an output table (after the select list has been processed) it can optionally be sorted. If sorting is not chosen, the rows will be returned in an unspecified order. The actual order in that case will depend on the scan and join plan types and the order on disk, but it must not be relied on. A particular output ordering can only be guaranteed if the sort step is explicitly chosen.

The ORDER BY clause specifies the sort order:

SELECT select_list
FROM table_expression
ORDER BY sort_expression [ASC | DESC]

The sort expression can be any expression that would be valid in the query’s select list. The expression can be followed by an optional ASC or DESC keyword to set the sort direction to ascending or descending. ASC order is the default. Ascending order puts smaller values first, where “smaller” is defined in terms of the < operator. Similarly, descending order is determined with the > operator. An example is:

SELECT a, b as x FROM table1 ORDER BY a;

Contoh penggunaannya seperti berikut:

Contoh lainnya seperti berikut:

Jika dijalankan maka hasilnya seperti berikut:

hr=# select location_id, department_id, department_name
hr-# from departments
hr-# order by location_id;
 location_id | department_id |   department_name
-------------+---------------+----------------------
        1400 |            60 | IT
        1500 |            50 | Shipping
        1700 |            30 | Purchasing
        1700 |           270 | Payroll
        1700 |            10 | Administration
        1700 |           230 | IT Helpdesk
        1800 |            20 | Marketing
        2400 |            40 | Human Resources
        2500 |            80 | Sales
        2700 |            70 | Public Relations
(28 rows)

The sort expression

A sort_expression can also be the column label or number of an output column, as in:

SELECT a, b, a + b as x FROM table1 ORDER BY x;
SELECT a, b, a + b as x FROM table1 ORDER BY 1;

Note that an output column name has to stand alone, that is, it cannot be used in an expression — for example, this is not correct:

SELECT a, b, a + b as x FROM table1 ORDER BY x + b;

This restriction is made to reduce ambiguity. There is still ambiguity if an ORDER BY item is a simple name that could match either an output column name or a column from the table expression.

ORDER BY can be applied to the result of a UNION, INTERSECT, or EXCEPT combination, but in this case it is only permitted to sort by output column names or numbers, not by expressions.

Contoh penggunaannya seperti berikut:

Jika dijalankan maka hasilnya seperti berikut:

hr=# select location_id, department_id, department_name
hr-# from departments
hr-# order by 2;
 location_id | department_id |   department_name
-------------+---------------+----------------------
        1700 |            10 | Administration
        1800 |            20 | Marketing
        1700 |            30 | Purchasing
        2400 |            40 | Human Resources
        1500 |            50 | Shipping
        1400 |            60 | IT
        2700 |            70 | Public Relations
        2500 |            80 | Sales
        1700 |            90 | Executive
        1700 |           100 | Finance
        1700 |           260 | Recruiting
        1700 |           270 | Payroll
        1700 |           300 | System Analis
(28 rows)

Using multiple columns to sort

ORDER BY clause can be more than one expression the later values are used to sort rows that are equal according to the earlier values.

SELECT select_list
FROM table_expression
ORDER BY    
    sort_expression1 [ASC | DESC]
    [, sort_expression2 [ASC | DESC] ...]

Note that the ordering options are considered independently for each sort column. For example ORDER BY x, y DESC means ORDER BY x ASC, y DESC, which is not the same as ORDER BY x DESC, y DESC.

Contoh penggunaannya seperti berikut:

Jika dijalankan maka hasilnya seperti berikut:

hr=# select location_id, department_id, department_name
hr-# from departments
hr-# order by 
    location_id asc, 
    department_id desc;
 location_id | department_id |   department_name
-------------+---------------+----------------------
        1400 |            60 | IT
        1500 |            50 | Shipping
        1700 |           170 | Manufacturing
        1700 |           160 | Benefits
        1700 |           150 | Shareholder Services
        1700 |           140 | Control And Credit
        1700 |           130 | Corporate Tax
        1700 |           120 | Treasury
        1700 |           110 | Accounting
        1700 |           100 | Finance
        1700 |            90 | Executive
        1700 |            30 | Purchasing
        1700 |            10 | Administration
        1800 |            20 | Marketing
        2400 |            40 | Human Resources
        2500 |            80 | Sales
(28 rows)

Choose Null value show at first or last?

The NULLS FIRST and NULLS LAST options can be used to determine whether nulls appear before or after non-null values in the sort ordering. By default, null values sort as if larger than any non-null value; that is, NULLS FIRST is the default for DESC order, and NULLS LAST otherwise.

SELECT select_list
FROM table_expression
ORDER BY    
    sort_expression1 [ASC | DESC] [NULLS { FIRST | LAST }]
    [, sort_expression2 [ASC | DESC] [NULLS { FIRST | LAST }] ...]

Contoh penggunaannya seperti berikut:

Jika dijalankan maka hasilnya seperti berikut:

hr=# select employee_id, first_name, salary, commission_pct
hr-# from employees
hr-# order by
hr-#   salary desc,
hr-#   commission_pct asc nulls first;
 employee_id | first_name  |  salary  | commission_pct
-------------+-------------+----------+----------------
         100 | Steven      | 24000.00 |
         102 | Lex         | 17000.00 |
         101 | Neena       | 17000.00 |
         145 | John        | 14000.00 |           0.40
         146 | Karen       | 13500.00 |           0.30
         201 | Michael     | 13000.00 |
         205 | Shelley     | 12000.00 |
         108 | Nancy       | 12000.00 |
         147 | Alberto     | 12000.00 |           0.30
         168 | Lisa        | 11500.00 |           0.25
         114 | Den         | 11000.00 |
         174 | Ellen       | 11000.00 |           0.30
         148 | Gerald      | 11000.00 |           0.30
(107 rows)

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