오라클 COUNT where - olakeul COUNT where

Summary: in this tutorial, you will learn how to use the Oracle COUNT() function to get the number of items in a group.

The Oracle COUNT() function is an aggregate function that returns the number of items in a group.

The syntax of the COUNT() function is as follows:

COUNT( [ALL | DISTINCT | * ] expression)
Code language: SQL (Structured Query Language) (sql)

The COUNT() function accepts a clause which can be either ALL, DISTINCT, or *:

• COUNT(*) function returns the number of items in a group, including NULL and duplicate values.
• COUNT(DISTINCT expression) function returns the number of unique and non-null items in a group.
• COUNT(ALL expression) evaluates the expression and returns the number of non-null items in a group, including duplicate values.

If you don’t explicitly specify DISTINCT or ALL, the COUNT() function uses the ALL by default.

Note that, unlike other aggregate functions such as AVG() and SUM(), the COUNT(*) function does not ignore NULL values.

Oracle COUNT() examples

Let’s take some examples of using the COUNT() function.

A) COUNT(*) vs. COUNT(DISTINCT expr) vs. COUNT(ALL)

Let’s create a table named items that consists of a val column and insert some sample data into the table for the demonstration.

CREATE TABLE items(val number);

INSERT INTO items(val) VALUES(1);
INSERT INTO items(val) VALUES(1);
INSERT INTO items(val) VALUES(2);
INSERT INTO items(val) VALUES(3);
INSERT INTO items(val) VALUES(NULL);
INSERT INTO items(val) VALUES(4);
INSERT INTO items(val) VALUES(NULL);

SELECT * FROM items;



Code language: SQL (Structured Query Language) (sql)

The following statement uses the COUNT(*) function to return the number of rows in the items table including NULL and duplicate values:

SELECT
    COUNT(*)
FROM
    items;
Code language: SQL (Structured Query Language) (sql)

The following statement uses the COUNT(DISTINCT val) to return only the number of distinct and non-null rows from the items table:

SELECT
    COUNT( DISTINCT val )
FROM
    items;
Code language: SQL (Structured Query Language) (sql)

The following statement uses the COUNT(ALL val) function to return the number of non-null rows in the items table, considering duplicates.

SELECT
    COUNT( ALL val )
FROM
    items;
Code language: SQL (Structured Query Language) (sql)

B) Simple Oracle COUNT() example

The following example returns the number of rows in the products table:

SELECT
    COUNT(*)
FROM
    products;
Code language: SQL (Structured Query Language) (sql)

C) Oracle COUNT() with WHERE clause example

If you want to find the number of products in the category id 1, you can add a WHERE clause to the query above:

SELECT
    COUNT(*)
FROM
    products
WHERE
    category_id = 1;
Code language: SQL (Structured Query Language) (sql)

D) Oracle COUNT() with GROUP BY clause example

To find the number of products in each product category, you use the following statement:

SELECT
    category_id,
    COUNT(*)
FROM
    products
GROUP BY
    category_id
ORDER BY
    category_id;



Code language: SQL (Structured Query Language) (sql)

In this example,

• First, the GROUP BY clause divides the products into groups based on the product category (category_id).
• Second, the COUNT(*) function returns the number of products for each group.

E) Oracle COUNT() with LEFT JOIN clause

The following examples get all category names and the number of products in each category by joining the product_categories with the products table and using the COUNT() function with the GROUP BY clause.

SELECT
    category_name,
    COUNT( product_id )
FROM
    product_categories
LEFT JOIN products
        USING(category_id)
GROUP BY
    category_name
ORDER BY
    category_name;  
Code language: SQL (Structured Query Language) (sql)

F) Oracle COUNT() with HAVING clause example

The following statement retrieves category names and the number of products in each. In addition, it uses a HAVING clause to return the only category whose the number of products is greater than 50.

SELECT
    category_name,
    COUNT( product_id )
FROM
    product_categories
LEFT JOIN products
        USING(category_id)
GROUP BY
    category_name
HAVING
    COUNT( product_id ) > 50
ORDER BY
    category_name;
Code language: SQL (Structured Query Language) (sql)

G) Using Oracle COUNT() and HAVING clause to find duplicate values

You can use the COUNT() function and a HAVING clause to find rows with duplicate values in a specified column.

For example, the following statement returns the contacts’ last names that appear more than one:

SELECT
    last_name,
    COUNT( last_name )
FROM
    contacts
GROUP BY
    last_name
HAVING
    COUNT( last_name )> 1
ORDER BY
    last_name;
Code language: SQL (Structured Query Language) (sql)

In this statement:

• Firstly, the GROUP BY clause divides the rows in the contacts table into groups based on the values in the last_name column.
• Secondly, the COUNT() function returns the number of the same last names for each last name.
• Finally, the HAVING clause returns only groups that have more than one value of the last name.

In this tutorial, you have learned how to use the Oracle COUNT() function to return the number of items in a group.

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