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Oracle ROW_NUMBER() examples
Using ROWNUM with Subqueries
参数Arguments
Example
- Recommended Way to Use ROWNUM
- Use ROWNUM to Limit Results
ROW_NUMBER Analytic Function
Setup
PostgreSQL ROW_NUMBER() function examples
MySQL ROW_NUMBER() function examples
Команды для создания запросов
SQL ROW_NUMBER() Function Overview
ПримерыExamples
SQL Учебник
Пример

Oracle ROW_NUMBER() examples

We’ll use the table from the sample database to demonstrate the function.

Oracle simple example

The following statement returns the row number, product name and list price from the table. The row number values are assigned based on the order of list prices.

The following picture shows the output:

This is a trivial example to just show how the function works.

To effectively use the function, you should use a subquery or a common table expression to retrieve row numbers for a specified range to get the top-N, bottom-N, and inner-N results.

Using Oracle function for pagination

The function is useful for pagination in applications.

Suppose you want to display products by pages with the list price from high to low, each page has 10 products. To display the third page, you use the function as follows:

The output is:

In this example, the CTE used the function to assign each row a sequential integer in descending order. The outer query retrieved the row whose row numbers are between 31 and 40.

Using Oracle function for the top-N query example

To get a single most expensive product by category, you can use the function as shown in the following query:

Here is the output:

In this example:

First, the clause divided the rows into partitions by category id.
Then, the clause sorted the products in each category by list prices in descending order.
Next, the function is applied to each row in a specific category id. It re-initialized the row number for each category.
After that, the outer query selected the rows with row number 1 which is the most expensive product in each category.

For the consistent result, the query must return a result set with the deterministic order. For example, if two products had the same highest prices, then the result would not be consistent. It could return the first or second product.

To get more than one product with the same N-highest prices, you can use the or function.

In this tutorial, you have learned how to use the Oracle function to make useful queries such as inner-N, top-N, and bottom-N.

Was this tutorial helpful?

Using ROWNUM with Subqueries

Offset-fetch в t-sql

A more appropriate way to use the ROWNUM pseudocolumn is with a subquery.

The basic steps are:

Write your query
Order your query
Enclose this query within a subquery
Filter the outer query using ROWNUM

Let’s see an example. Say we wanted to see the students who are in the top 5 when ordered by last_name.

One way to do this might be:

Result:

ROWNUM	FIRST_NAME	LAST_NAME	ADDRESS_STATE
3	Tom	Capper	Nevada
4	Mark	Holloway	New York
2	Susan	Johnson	Colorado
1	John	Smith	New York
5	Steven	Webber	New York

However, this is not correct, because the ROWNUM is used to limit the number of rows, and then the ORDER BY is done. This shows the top 5 in the table before the ORDER BY.

To do this correctly, enclose it in a subquery as mentioned earlier.

Result:

ROWNUM	FIRST_NAME	LAST_NAME	ADDRESS_STATE
1	Mark	Anderson	California
2	Julie	Armstrong	Texas
3	Tom	Capper	Nevada
4	Andrew	Cooper	Texas
5	Tanya	Hall	Texas

This now shows the student records, ordered by last_name, and then only the top 5, which is the result we were looking for.

So, that’s what Oracle ROWNUM and Oracle ROW_NUMBER does, and how they are different. Be careful when you use the ORDER BY as well, as it may not give you the results you need.

Lastly, if you enjoy the information and career advice I’ve been providing, sign up to my newsletter below to stay up-to-date on my articles. You’ll also receive a fantastic bonus. Thanks!

参数Arguments

Oracle / plsql: foreign keys with set null on delete

PARTITION BY value_expression PARTITION BY value_expression将 FROM 子句生成的结果集划分为应用 ROW_NUMBER 函数的分区。Divides the result set produced by the FROM clause into partitions to which the ROW_NUMBER function is applied. value_expression 指定对结果集进行分区所依据的列。value_expression specifies the column by which the result set is partitioned. 如果未指定，则此函数将查询结果集的所有行视为单个组。If is not specified, the function treats all rows of the query result set as a single group. 有关详细信息，请参阅 OVER 子句 (Transact-SQL)。For more information, see OVER Clause (Transact-SQL).

order_by_clause order_by_clause 子句可确定在特定分区中为行分配唯一的顺序。The clause determines the sequence in which the rows are assigned their unique within a specified partition. 它是必需的。It is required. 有关详细信息，请参阅 OVER 子句 (Transact-SQL)。For more information, see OVER Clause (Transact-SQL).

Example

Секционирование таблиц в oracle на практике

Let’s look at some Oracle ROWNUM function examples and explore how to use the ROWNUM function in Oracle/PLSQL.

In this ROWNUM example, we have a table called customers with the following data:

CUSTOMER_ID  LAST_NAME  FIRST_NAME  FAVORITE_WEBSITE
-----------  ---------  ----------  ---------------------
       4000  Jackson    Joe         www.techonthenet.com
       5000  Smith      Jane        www.digminecraft.com
       6000  Ferguson   Samantha    www.bigactivities.com
       7000  Reynolds   Allen       www.checkyourmath.com
       8000  Anderson   Paige
       9000  Johnson    Derek       www.techonthenet.com

Now let’s demonstrate how the ROWNUM function works by selecting data from the customers table. Enter the following SQL statement in Oracle:

SELECT ROWNUM, customers.*
FROM customers
WHERE customer_id > 4500;

These are the results that you should see:

ROWNUM  CUSTOMER_ID  LAST_NAME  FIRST_NAME  FAVORITE_WEBSITE
------  -----------  ---------  ----------  ---------------------
     1         5000  Smith      Jane        www.digminecraft.com
     2         6000  Ferguson   Samantha    www.bigactivities.com
     3         7000  Reynolds   Allen       www.checkyourmath.com
     4         8000  Anderson   Paige
     5         9000  Johnson    Derek       www.techonthenet.com

In this example, the ROWNUM function returns 1 for the first record, 2 for the second record, and so on. Since this is a very simple example, it would appear that the ROWNUM function is straight-forward to use, but it is a bit more complicated than you think.

Let’s complicate the example by introducing an ORDER BY clause and sort the results by last_name in ascending order. Enter the following SELECT statement in Oracle:

SELECT ROWNUM, customers.*
FROM customers
WHERE customer_id > 4500
ORDER BY last_name;

You should see these results:

ROWNUM   CUSTOMER_ID   LAST_NAME   FIRST_NAME   FAVORITE_WEBSITE
------   -----------   ---------   ----------   ---------------------
     4          8000   Anderson    Paige
     2          6000   Ferguson    Samantha     www.bigactivities.com
     5          9000   Johnson     Derek        www.techonthenet.com
     3          7000   Reynolds    Allen        www.checkyourmath.com
     1          5000   Smith       Jane         www.digminecraft.com

You would expect that the first row in your result set would have a ROWNUM value of 1, but in this example, it has a ROWNUM value of 4. Why is this? Well, it depends how Oracle accessed the rows in the query. For example your results can vary depending on a lot of factors (ie: the order that you inserted the data in the table or if there is an index on the table).

Because of these factors, there is a right and wrong way to use the ROWNUM function.

Recommended Way to Use ROWNUM

The most reliable way to use the ROWNUM is to use a subquery to filter and sort your results and then place the ROWNUM function in the outer SELECT. Enter the following SELECT in Oracle:

SELECT ROWNUM, a.*
FROM (SELECT customers.*
      FROM customers
      WHERE customer_id > 4500
      ORDER BY last_name) a;

These are the results that you should see:

ROWNUM  CUSTOMER_ID  LAST_NAME  FIRST_NAME  FAVORITE_WEBSITE
------  -----------  ---------  ----------  ---------------------
     1         8000  Anderson   Paige
     2         6000  Ferguson   Samantha    www.bigactivities.com
     3         9000  Johnson    Derek       www.techonthenet.com
     4         7000  Reynolds   Allen       www.checkyourmath.com
     5         5000  Smith      Jane        www.digminecraft.com

By using a subquery in this way, it forces the ROWNUM to properly order the records, starting at 1 for the first record, 2 for the second and so on.

Use ROWNUM to Limit Results

The ROWNUM function is also handy if you want to limit the results of a query. For example, you could return the top 2 results. Enter the following SQL statement in Oracle:

SELECT *
FROM (SELECT customers.*
      FROM customers
      WHERE customer_id > 4500
      ORDER BY last_name)
WHERE ROWNUM < 3;

You should see these results:

CUSTOMER_ID  LAST_NAME  FIRST_NAME  FAVORITE_WEBSITE
-----------  ---------  ----------  ---------------------
       8000  Anderson   Paige
       6000  Ferguson   Samantha    www.bigactivities.com

In this example, the ROWNUM function would return the top 2 results because we want .

If we wanted to get the bottom 2 results, we could just change the sort order of the subquery to . Enter the following query in Oracle:

SELECT *
FROM (SELECT customers.*
      FROM customers
      WHERE customer_id > 4500
      ORDER BY last_name DESC)
WHERE ROWNUM < 3;

These are the results that you should see:

CUSTOMER_ID  LAST_NAME  FIRST_NAME  FAVORITE_WEBSITE
-----------  ---------  ----------  -------------------
       5000  Smith      Jane        www.digminecraft.com
       7000  Reynolds   Allen       www.checkyourmath.com

Now we get the bottom 2 results because we have sorted the last_name in descending order.

ROW_NUMBER Analytic Function

If you have ever used the pseudocolumn, you will have an idea what the analytic function does. It is used to assign a unique number from 1-N to the rows within a partition. At first glance this may seem similar to the RANK and DENSE_RANK analytic functions, but the function ignores ties and always gives a unique number to each row.

The basic description for the analytic function is shown below. The analytic clause is described in more detail .

ROW_NUMBER() OVER ( order_by_clause)

The analytic function is order-sensitive and produces an error if you attempt to use it without an in the analytic clause. Unlike some other analytic functions, it doesn’t support the windowing clause. Omitting a partitioning clause from the clause means the whole result set is treated as a single partition. In the following example we assign a unique row number to each employee based on their salary (lowest to highest). The example also includes and to show the difference in how ties are handled.

SELECT empno,
       ename,
       deptno,
       sal,
       ROW_NUMBER() OVER (ORDER BY sal) AS row_num,
       RANK() OVER (ORDER BY sal) AS row_rank,
       DENSE_RANK() OVER (ORDER BY sal) AS row_dense_rank
FROM   emp;

     EMPNO ENAME          DEPTNO        SAL    ROW_NUM   ROW_RANK ROW_DENSE_RANK
---------- ---------- ---------- ---------- ---------- ---------- --------------
      7369 SMITH              20        800          1          1              1
      7900 JAMES              30        950          2          2              2
      7876 ADAMS              20       1100          3          3              3
      7521 WARD               30       1250          4          4              4
      7654 MARTIN             30       1250          5          4              4
      7934 MILLER             10       1300          6          6              5
      7844 TURNER             30       1500          7          7              6
      7499 ALLEN              30       1600          8          8              7
      7782 CLARK              10       2450          9          9              8
      7698 BLAKE              30       2850         10         10              9
      7566 JONES              20       2975         11         11             10
      7788 SCOTT              20       3000         12         12             11
      7902 FORD               20       3000         13         12             11
      7839 KING               10       5000         14         14             12

SQL>

Adding the partitioning clause allows us to assign the row number within a partition. In the following example we assign the row number within the department, based on highest to lowest salary.

SELECT empno,
       ename,
       deptno,
       sal,
       ROW_NUMBER() OVER (PARTITION BY deptno ORDER BY sal DESC) AS row_num
FROM   emp;

     EMPNO ENAME          DEPTNO        SAL    ROW_NUM
---------- ---------- ---------- ---------- ----------
      7839 KING               10       5000          1
      7782 CLARK              10       2450          2
      7934 MILLER             10       1300          3
      7788 SCOTT              20       3000          1
      7902 FORD               20       3000          2
      7566 JONES              20       2975          3
      7876 ADAMS              20       1100          4
      7369 SMITH              20        800          5
      7698 BLAKE              30       2850          1
      7499 ALLEN              30       1600          2
      7844 TURNER             30       1500          3
      7654 MARTIN             30       1250          4
      7521 WARD               30       1250          5
      7900 JAMES              30        950          6

SQL>

This allows us to write Top-N queries at the partition level. The following example brings back the best paid person in each department, ignoring ties.

SELECT *
FROM   (SELECT empno,
               ename,
               deptno,
               sal,
               ROW_NUMBER() OVER (PARTITION BY deptno ORDER BY sal DESC) AS row_num
        FROM   emp)
WHERE  row_num = 1;

     EMPNO ENAME          DEPTNO        SAL    ROW_NUM
---------- ---------- ---------- ---------- ----------
      7839 KING               10       5000          1
      7788 SCOTT              20       3000          1
      7698 BLAKE              30       2850          1

SQL>

Setup

The examples in this article require the following table.

--DROP TABLE emp PURGE;

CREATE TABLE emp (
  empno    NUMBER(4) CONSTRAINT pk_emp PRIMARY KEY,
  ename    VARCHAR2(10),
  job      VARCHAR2(9),
  mgr      NUMBER(4),
  hiredate DATE,
  sal      NUMBER(7,2),
  comm     NUMBER(7,2),
  deptno   NUMBER(2)
);

INSERT INTO emp VALUES (7369,'SMITH','CLERK',7902,to_date('17-12-1980','dd-mm-yyyy'),800,NULL,20);
INSERT INTO emp VALUES (7499,'ALLEN','SALESMAN',7698,to_date('20-2-1981','dd-mm-yyyy'),1600,300,30);
INSERT INTO emp VALUES (7521,'WARD','SALESMAN',7698,to_date('22-2-1981','dd-mm-yyyy'),1250,500,30);
INSERT INTO emp VALUES (7566,'JONES','MANAGER',7839,to_date('2-4-1981','dd-mm-yyyy'),2975,NULL,20);
INSERT INTO emp VALUES (7654,'MARTIN','SALESMAN',7698,to_date('28-9-1981','dd-mm-yyyy'),1250,1400,30);
INSERT INTO emp VALUES (7698,'BLAKE','MANAGER',7839,to_date('1-5-1981','dd-mm-yyyy'),2850,NULL,30);
INSERT INTO emp VALUES (7782,'CLARK','MANAGER',7839,to_date('9-6-1981','dd-mm-yyyy'),2450,NULL,10);
INSERT INTO emp VALUES (7788,'SCOTT','ANALYST',7566,to_date('13-JUL-87','dd-mm-rr')-85,3000,NULL,20);
INSERT INTO emp VALUES (7839,'KING','PRESIDENT',NULL,to_date('17-11-1981','dd-mm-yyyy'),5000,NULL,10);
INSERT INTO emp VALUES (7844,'TURNER','SALESMAN',7698,to_date('8-9-1981','dd-mm-yyyy'),1500,0,30);
INSERT INTO emp VALUES (7876,'ADAMS','CLERK',7788,to_date('13-JUL-87', 'dd-mm-rr')-51,1100,NULL,20);
INSERT INTO emp VALUES (7900,'JAMES','CLERK',7698,to_date('3-12-1981','dd-mm-yyyy'),950,NULL,30);
INSERT INTO emp VALUES (7902,'FORD','ANALYST',7566,to_date('3-12-1981','dd-mm-yyyy'),3000,NULL,20);
INSERT INTO emp VALUES (7934,'MILLER','CLERK',7782,to_date('23-1-1982','dd-mm-yyyy'),1300,NULL,10);
COMMIT;

PostgreSQL ROW_NUMBER() function examples

We will use the table created in the PostgreSQL window function tutorial to demonstrate the functionality of the function.

The following is the data in the table:

See the following query.

Because we did not use the clause, the function considers the whole result set as a partition.

The clause sorts the result set by , therefore, the function assigns integer values to the rows based on the order.

In the following query, we change the column in the clause to product_name, the function assigns the integer values to each row based on the product name order.

In the following query, we use the clause to divide the window into subsets based on the values in the column. In this case, the function assigns one to the starting row of each partition and increases by one for the next row within the same partition.

The clause sorts the rows in each partition by the values in the column.

MySQL ROW_NUMBER() function examples

Let’s use the table from the sample database for the demonstration:

1) Assigning sequential numbers to rows

The following statement uses the function to assign a sequential number to each row from the table:

Here is the output:

2) Finding top N rows of every group

You can use the function for the queries that find the top N rows for every group, for example, top three sales employees of every sales channel, top five high-performance products of every category.

The following statement finds the top three products that have the highest inventory of every product line:

In this example,

First, we used the function to rank the inventory of all products in each product line by partitioning all products by product line and ordering them by quantity in stock in descending order. As the result, each product is assigned a rank based on its quantity in stock. and the rank is reset for each product line.
Then, we selected only products whose rank is less than or equal to three.

The following shows the output:

3) Removing duplicate rows

You can use the to turn non-unique rows into unique rows and then delete the duplicate rows. Consider the following example.

First, create a table with some duplicate values:

Second, use the function to divide the rows into partitions by all columns. The row number will restart for each unique set of rows.

As you can see from the output, the unique rows are the ones whose the row number equals one.

Third, you can use the common table expression (CTE) to return the duplicate rows and delete statement to remove:

Notice that the MySQL does not support CTE based delete, therefore, we had to join the original table with the CTE as a workaround.

4) Pagination using function

Because the assigns each row in the result set a unique number, you can use it for pagination.

Suppose, you need to display a list of products with 10 products per page. To get the products for the second page, you use the following query:

Here is the output:

In this tutorial, you have learned how to use the MySQL function to generate a sequential number for each row in a result set.

Was this tutorial helpful?

Команды для создания запросов

13. SELECT

используется для получения данных из определённой таблицы:

Следующей командой можно вывести все данные из таблицы:

14. SELECT DISTINCT

В столбцах таблицы могут содержаться повторяющиеся данные. Используйте для получения только неповторяющихся данных.

15. WHERE

Можно использовать ключевое слово в для указания условий в запросе:

В запросе можно задавать следующие условия:

сравнение текста;
сравнение численных значений;
логические операции AND (и), OR (или) и NOT (отрицание).

Попробуйте выполнить следующие команды

Обратите внимание на условия, заданные в :

16. GROUP BY

Оператор часто используется с агрегатными функциями, такими как , , , и , для группировки выходных значений.

Выведем количество курсов для каждого факультета:

17. HAVING

Ключевое слово было добавлено в SQL потому, что не может быть использовано для работы с агрегатными функциями.

Выведем список факультетов, у которых более одного курса:

18. ORDER BY

используется для сортировки результатов запроса по убыванию или возрастанию. отсортирует по возрастанию, если не будет указан способ сортировки или .

Выведем список курсов по возрастанию и убыванию количества кредитов:

19. BETWEEN

используется для выбора значений данных из определённого промежутка. Могут быть использованы числовые и текстовые значения, а также даты.

Выведем список инструкторов, чья зарплата больше 50 000, но меньше 100 000:

20. LIKE

Оператор используется в , чтобы задать шаблон поиска похожего значения.

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События и курсы на tproger.ru

Есть два свободных оператора, которые используются в :

(ни одного, один или несколько символов);
(один символ).

Выведем список курсов, в имени которых содержится , и список курсов, название которых начинается с :

21. IN

С помощью можно указать несколько значений для оператора :

Выведем список студентов с направлений Comp. Sci., Physics и Elec. Eng.:

22. JOIN

используется для связи двух или более таблиц с помощью общих атрибутов внутри них. На изображении ниже показаны различные способы объединения в SQL

Обратите внимание на разницу между левым внешним объединением и правым внешним объединением:

Выведем список всех курсов вне зависимости от того, обязательны они или нет:

23. View

— это виртуальная таблица SQL, созданная в результате выполнения выражения. Она содержит строки и столбцы и очень похожа на обычную SQL-таблицу. всегда показывает самую свежую информацию из базы данных.

Создадим , состоящую из курсов с 3 кредитами:

24. Агрегатные функции

Эти функции используются для получения совокупного результата, относящегося к рассматриваемым данным. Ниже приведены общеупотребительные агрегированные функции:

— возвращает количество строк;
— возвращает сумму значений в данном столбце;
— возвращает среднее значение данного столбца;
— возвращает наименьшее значение данного столбца;
— возвращает наибольшее значение данного столбца.

SQL ROW_NUMBER() Function Overview

The is a window function that assigns a sequential integer number to each row in the query’s result set.

The following illustrates the syntax of the function:

In this syntax,

First, the clause divides the result set returned from the clause into partitions. The clause is optional. If you omit it, the whole result set is treated as a single partition.
Then, the clause sorts the rows in each partition. Because the is an order sensitive function, the clause is required.
Finally, each row in each partition is assigned a sequential integer number called a row number. The row number is reset whenever the partition boundary is crossed.

ПримерыExamples

A.A. Простые примерыSimple examples

Приведенный ниже запрос возвращает четыре системные таблицы в алфавитном порядке.The following query returns the four system tables in alphabetic order.