SQL SELECT with DISTINCT on multiple columns

DISTINCT on multiple columns

In SQL multiple fields may also be added with DISTINCT clause. DISTINCT will eliminate those rows where all the selected fields are identical.

Example: Sample SELECT statement

This query retrieves specific columns from the orders table, filtered by agent_code='A002'

SQL Code:

SELECT agent_code, ord_amount, cust_code, ord_num
-- Select specific columns: agent_code, ord_amount, cust_code, ord_num
FROM orders 
-- From the table 'orders'
WHERE agent_code='A002';
-- Filter the results to only include rows where the 'agent_code' is 'A002'

Explanation:

• SELECT agent_code, ord_amount, cust_code, ord_num: This line specifies the columns that you want to retrieve data from. It selects the columns 'agent_code', 'ord_amount', 'cust_code', and 'ord_num' from the 'orders' table.


• FROM orders: This line specifies the table from which you want to retrieve data. In this case, it's the 'orders' table.


• WHERE agent_code='A002': This line specifies a condition for filtering the results. It filters the results to only include rows where the value in the 'agent_code' column is 'A002'. This condition acts as a filter, allowing only rows with 'A002' as the agent code to be included in the result set.

Relational Algebra Expression:

Relational Algebra Tree:

Sample table: orders

Output:

AGENT_CODE ORD_AMOUNT CUST_CODE     ORD_NUM
---------- ---------- ---------- ----------
A002             2500 C00005         200106
A002              500 C00022         200123
A002              500 C00009         200120
A002              500 C00022         200126
A002             3500 C00009         200128
A002             1200 C00009         200133
A002             4000 C00022         200113

The above result shows the same agent_code, ord_amount and cust_code appears more than once in theorders table.

Example: SELECT with DISTINCT on two columns

The SQL statement below retrieves identical rows based on the 'agent_code' and 'ord_amount' columns from the orders table :

SQL Code:

SELECT DISTINCT agent_code, ord_amount
-- Select distinct combinations of 'agent_code' and 'ord_amount'
FROM orders 
-- From the table 'orders'
WHERE agent_code='A002';
-- Filter the results to only include rows where the 'agent_code' is 'A002'

Explanation:

• SELECT DISTINCT agent_code, ord_amount: This line specifies that you want to retrieve unique combinations of 'agent_code' and 'ord_amount'. The DISTINCT keyword ensures that only unique combinations are returned; any duplicate combinations will be eliminated.


• FROM orders: This line specifies the table from which you want to retrieve data. In this case, it's the 'orders' table.


• WHERE agent_code='A002': This line specifies a condition for filtering the results. It filters the results to only include rows where the value in the 'agent_code' column is 'A002'. This condition acts as a filter, allowing only rows with 'A002' as the agent code to be included in the result set.

Relational Algebra Expression:

Relational Algebra Tree:

Output:

AGENT_CODE ORD_AMOUNT
---------- ----------
A002             3500
A002             1200
A002             4000
A002              500
A002             2500

Pictorial presentation:

Example: SELECT with DISTINCT on three columns

The following SQL statement can be used to retrieve identical rows based on the 'agent_code', 'ord_amount', and 'cust_code' columns once from the orders table:

SQL Code:

SELECT DISTINCT agent_code, ord_amount, cust_code
-- Select distinct combinations of 'agent_code', 'ord_amount', and 'cust_code'
FROM orders 
-- From the table 'orders'
WHERE agent_code='A002';
-- Filter the results to only include rows where the 'agent_code' is 'A002'

Explanation:

• SELECT DISTINCT agent_code, ord_amount, cust_code: This line specifies that you want to retrieve unique combinations of 'agent_code', 'ord_amount', and 'cust_code'. The DISTINCT keyword ensures that only unique combinations are returned; any duplicate combinations will be eliminated.


• FROM orders: This line specifies the table from which you want to retrieve data. In this case, it's the 'orders' table.


• WHERE agent_code='A002': This line specifies a condition for filtering the results. It filters the results to only include rows where the value in the 'agent_code' column is 'A002'. This condition acts as a filter, allowing only rows with 'A002' as the agent code to be included in the result set.

Relational Algebra Expression:

Relational Algebra Tree:

Output:

AGENT_CODE ORD_AMOUNT CUST_CODE
---------- ---------- ----------
A002              500 C00022
A002             3500 C00009
A002             2500 C00005
A002              500 C00009
A002             4000 C00022
A002             1200 C00009

Pictorial presentation:

Example : SELECT with DISTINCT on all columns of the first query

To get the identical rows (on four columns agent_code, ord_amount, cust_code, and ord_num) once from the orders table , the following SQL statement can be used :

SQL Code:

SELECT DISTINCT agent_code, ord_amount, cust_code, ord_num
-- Select distinct combinations of 'agent_code', 'ord_amount', 'cust_code', and 'ord_num'
FROM orders 
-- From the table 'orders'
WHERE agent_code='A002';
-- Filter the results to only include rows where the 'agent_code' is 'A002'

Explanation:

• SELECT DISTINCT agent_code, ord_amount, cust_code, ord_num: This line specifies that you want to retrieve unique combinations of 'agent_code', 'ord_amount', 'cust_code', and 'ord_num'. The DISTINCT keyword ensures that only unique combinations are returned; any duplicate combinations will be eliminated.


• FROM orders: This line specifies the table from which you want to retrieve data. In this case, it's the 'orders' table.


• WHERE agent_code='A002': This line specifies a condition for filtering the results. It filters the results to only include rows where the value in the 'agent_code' column is 'A002'. This condition acts as a filter, allowing only rows with 'A002' as the agent code to be included in the result set.

Relational Algebra Expression:

Relational Algebra Tree:

Output:

AGENT_CODE ORD_AMOUNT CUST_CODE     ORD_NUM
---------- ---------- ---------- ----------
A002              500 C00022         200126
A002             2500 C00005         200106
A002              500 C00009         200120
A002             1200 C00009         200133
A002             4000 C00022         200113
A002             3500 C00009         200128
A002              500 C00022         200123

In the above output, all rows whose agent_code is 'A002' have returned because there is no identical rows on agent_code, ord_amount, cust_code and ord_num. See the following presentation :

Pictorial presentation:

SELECT with DISTINCT on multiple columns and ORDER BY clause

You can use an order by clause in the select statement with distinct on multiple columns. Here is an example:

SQL Code:

SELECT DISTINCT agent_code, ord_amount
-- Select distinct combinations of 'agent_code' and 'ord_amount'
FROM orders 
-- From the table 'orders'
WHERE agent_code='A002' 
-- Filter the results to only include rows where the 'agent_code' is 'A002'
ORDER BY ord_amount;
-- Sort the results in ascending order based on the 'ord_amount' column

Explanation:

• SELECT DISTINCT agent_code, ord_amount: This line specifies that you want to retrieve unique combinations of 'agent_code' and 'ord_amount'. The DISTINCT keyword ensures that only unique combinations are returned; any duplicate combinations will be eliminated.


• FROM orders: This line specifies the table from which you want to retrieve data. In this case, it's the 'orders' table.


• WHERE agent_code='A002': This line specifies a condition for filtering the results. It filters the results to only include rows where the value in the 'agent_code' column is 'A002'. This condition acts as a filter, allowing only rows with 'A002' as the agent code to be included in the result set.


• ORDER BY ord_amount: This line specifies how the results should be sorted. It sorts the results in ascending order based on the 'ord_amount' column. This means that the rows will be arranged from the smallest to the largest 'ord_amount'.

Relational Algebra Expression:

Relational Algebra Tree:

Output:

AGENT_CODE ORD_AMOUNT
---------- ----------
A002              500
A002             1200
A002             2500
A002             3500
A002             4000

Pictorial presentation:

COUNT() function and SELECT with DISTINCT on multiple columns

You can use the count() function in a select statement with distinct on multiple columns to count the distinct rows. Here is an example:

SELECT COUNT(*) 
-- Count the number of rows in the result set
FROM (
    SELECT DISTINCT agent_code, ord_amount, cust_code
    -- Select distinct combinations of 'agent_code', 'ord_amount', and 'cust_code'
    FROM orders 
    -- From the table 'orders'
    WHERE agent_code='A002'
    -- Filter the results to only include rows where the 'agent_code' is 'A002'
);

Explanation:

• SELECT COUNT(*): This line specifies that you want to count the number of rows in the result set.


• FROM (...): This line wraps the inner query in parentheses and treats it as a subquery. The subquery selects distinct combinations of 'agent_code', 'ord_amount', and 'cust_code' from the 'orders' table where the 'agent_code' is 'A002'.


• The inner query:


• SELECT DISTINCT agent_code, ord_amount, cust_code: Selects distinct combinations of 'agent_code', 'ord_amount', and 'cust_code' from the 'orders' table.


• FROM orders: Specifies the table from which you want to retrieve data. In this case, it's the 'orders' table.


• WHERE agent_code='A002': Filters the results to only include rows where the value in the 'agent_code' column is 'A002'.

Output:

  COUNT(*)
----------
         6

Pictorial presentation:

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