Как найти максимум в столбце sql - Исправление недочетов и поиск решений вместе с Examum.ru

Let’s say I have the following data in the Customers table: (nothing more)

ID   FirstName   LastName
-------------------------------
20   John        Mackenzie
21   Ted         Green
22   Marcy       Nate

What sort of SELECT statement can get me the number 22, in the ID column?

I need to do something like this to generate a unique ID. Sure I can let the system do this via auto-increment, but then how would I get the auto generated ID?

I thought of SELECT ID FROM Customers and counting the rows returned but this seems horribly inefficient, and in this case, it will incorrectly return «3», though I need a unique ID of 23.

Kevin

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asked Oct 10, 2009 at 5:14

Robin RodricksRobin Rodricks

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You can do

SELECT MAX(ID) FROM Customers;

answered Oct 10, 2009 at 5:15

If you’ve just inserted a record into the Customers table and you need the value of the recently populated ID field, you can use the SCOPE_IDENTITY function. This is only useful when the INSERT has occurred within the same scope as the call to SCOPE_IDENTITY.

INSERT INTO Customers(ID, FirstName, LastName)
Values
(23, 'Bob', 'Smith')

SET @mostRecentId = SCOPE_IDENTITY()

This may or may not be useful for you, but it’s a good technique to be aware of. It will also work with auto-generated columns.

Kevin

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answered Oct 10, 2009 at 5:21

David AndresDavid Andres

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SELECT * FROM Customers ORDER BY ID DESC LIMIT 1

Then get the ID.

McGarnagle

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answered Oct 17, 2012 at 3:37

select max(id) from customers

warren

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answered Oct 10, 2009 at 5:15

deostrolldeostroll

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To get it at any time, you can do SELECT MAX(Id) FROM Customers .

In the procedure you add it in, however, you can also make use of SCOPE_IDENTITY — to get the id last added by that procedure.
This is safer, because it will guarantee you get your Id—just in case others are being added to the database at the same time.

Pritesh Jain

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answered Oct 10, 2009 at 5:19

BrisbeBrisbe

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If you’re talking MS SQL, here’s the most efficient way. This retrieves the current identity seed from a table based on whatever column is the identity.

select IDENT_CURRENT('TableName') as LastIdentity

Using MAX(id) is more generic, but for example I have an table with 400 million rows that takes 2 minutes to get the MAX(id). IDENT_CURRENT is nearly instantaneous…

answered Oct 10, 2009 at 17:34

DamonDamon

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select max(id) from Customers

warren

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answered Oct 10, 2009 at 5:16

monksymonksy

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If you are using AUTOINCREMENT, use:

SELECT LAST_INSERT_ID();

Assumming that you are using Mysql:
http://dev.mysql.com/doc/refman/5.0/en/example-auto-increment.html

Postgres handles this similarly via the currval(sequence_name) function.

Note that using MAX(ID) is not safe, unless you lock the table, since it’s possible (in a simplified case) to have another insert that occurs before you call MAX(ID) and you lose the id of the first insert. The functions above are session based so if another session inserts you still get the ID that you inserted.

ItsJ0el

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answered Oct 10, 2009 at 5:33

If you’re not using auto-incrementing fields, you can achieve a similar result with something like the following:

insert into Customers (ID, FirstName, LastName)
    select max(ID)+1, 'Barack', 'Obama' from Customers;

This will ensure there’s no chance of a race condition which could be caused by someone else inserting into the table between your extraction of the maximum ID and your insertion of the new record.

This is using standard SQL, there are no doubt better ways to achieve it with specific DBMS’ but they’re not necessarily portable (something we take very seriously in our shop).

answered Oct 10, 2009 at 6:53

paxdiablopaxdiablo

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You can also use relational algebra. A bit lengthy procedure, but here it is just to understand how MAX() works:

E := π_ID (Table_Name)
E1 := π_ID (σ_{ID >= ID’} ((ρ_ID’ E) ⋈ E)) – π_ID (σ_{ID < ID’} ((ρ_ID’ E) ⋈ E))

Your answer:
Table_Name ⋈ E1

Basically what you do is subtract set of ordered relation(a,b) in which a<b from A where a, b ∈ A.

For relation algebra symbols see:
Relational algebra
From Wikipedia

answered Jan 24, 2013 at 17:33

ArnabArnab

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Here’s how I would make the next ID:

INSERT INTO table_name (
            ID,
            FIRSTNAME,
            SURNAME) 
            VALUES ((( 
                    SELECT COALESCE(MAX(B.ID)+1,1) AS NEXTID 
                        FROM table_name B
                        )), John2, Smith2);

With this you can make sure that even if the table ID is NULL, it will still work perfectly.

answered Jan 12, 2021 at 21:20

Depends on what SQL implementation you are using. Both MySQL and SQLite, for example, have ways to get last insert id. In fact, if you’re using PHP, there’s even a nifty function for exactly that mysql_insert_id().

You should probably look to use this MySQL feature instead of looking at all the rows just to get the biggest insert ID. If your table gets big, that could become very inefficient.

answered Oct 10, 2009 at 6:26

sohumsohum

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If you want to just select the id use select max(id) from customer.

If you want to select the entire row then use a query like this:

select c1.*
from customer c1, (select max(id) as max_id from customer )c2 
where c1.id=c2.max_id

c2 is an alias for the new temporary table which contains max id. Then its cross product is taken with customer table to get the entire row.

Here we are writing a query in the from clause, which can often be quite useful.

answered Jul 7, 2018 at 19:48

To find the next (still not used) auto-increment, I am using this function for somewhat years now.

public function getNewAI($table)
    {
        $newAI = false;

        $mysqli = new mysqli(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);
        if(mysqli_connect_errno()) {
            echo "Failed to connect to MySQL: " . mysqli_connect_error();
        }

        $sql = "SHOW TABLE STATUS LIKE '".$table."'";

        $result = $mysqli->query($sql);

        if($result) {
            $row = $result->fetch_assoc();

            $newAI = $row['Auto_increment'];
        }
        $mysqli->close();

        return $newAI;
    }

answered Dec 4, 2019 at 14:55

select * from tablename order by ID DESC

that will give you row with id 22

Ben

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answered Aug 23, 2012 at 6:02

In PHP:

$sql = mysql_query("select id from customers order by id desc");
$rs = mysql_fetch_array($sql);
if ( !$rs ) { $newid = 1; } else { $newid = $rs[newid]+1; }

thus $newid = 23 if last record in column id was 22.

skuntsel

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answered Feb 13, 2015 at 13:27

Источник

This SQL tutorial explains how to use the SQL MAX function with syntax and examples.

Description

The SQL MAX function is used to return the maximum value of an expression in a SELECT statement.

Syntax

The syntax for the MAX function in SQL is:

SELECT MAX(aggregate_expression)
FROM tables
[WHERE conditions];

OR the syntax for the MAX function when grouping the results by one or more columns is:

SELECT expression1, expression2, ... expression_n,
       MAX(aggregate_expression)
FROM tables
[WHERE conditions]
GROUP BY expression1, expression2, ... expression_n;

Parameters or Arguments

expression1, expression2, … expression_n: Expressions that are not encapsulated within the MAX function and must be included in the GROUP BY clause at the end of the SQL statement.
aggregate_expression: This is the column or expression from which the maximum value will be returned.
tables: The tables that you wish to retrieve records from. There must be at least one table listed in the FROM clause.
WHERE conditions: Optional. These are conditions that must be met for the records to be selected.

Example — With Single Expression

The simplest way to use the SQL MAX function would be to return a single field that calculates the MAX value.

For example, you might wish to know the maximum salary of all employees.

SELECT MAX(salary) AS "Highest salary"
FROM employees;

In this SQL MAX function example, we’ve aliased the MAX(salary) field as «Highest salary». As a result, «Highest salary» will display as the field name when the result set is returned.

Example — Using SQL GROUP BY Clause

In some cases, you will be required to use the SQL GROUP BY clause with the SQL MAX function.

For example, you could also use the SQL MAX function to return the name of each department and the maximum salary in the department.

SELECT department, MAX(salary) AS "Highest salary"
FROM employees
GROUP BY department;

Because you have listed one column in your SQL SELECT statement that is not encapsulated in the MAX function, you must use the SQL GROUP BY clause. The department field must, therefore, be listed in the GROUP BY section.

Frequently Asked Questions

Question: I’m trying to pull some info out of a table. To simplify, let’s say the table (report_history) has 4 columns: user_name, report_job_id, report_name, and report_run_date.

Each time a report is run in Oracle, a record is written to this table noting the above info. What I am trying to do is pull from this table when the last time each distinct report was run and who ran it last.

My initial query:

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

runs fine. However, it does not provide the name of the user who ran the report.

Adding user_name to both the select list and to the group by clause returns multiple lines for each report; the results show the last time each person ran each report in question. (i.e. User1 ran Report 1 on 01-JUL-03, User2 ran Report1 on 01-AUG-03). I don’t want that….I just want to know who ran a particular report the last time it was run.

Any suggestions?

Answer: This is where things get a bit complicated. The SQL SELECT statement below will return the results that 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;

Let’s take a few moments to explain what we’ve done.

First, we’ve aliased the first instance of the report_history table as rh.

Second, we’ve included two components in our FROM clause. The first is the table called report_history (aliased as rh). The second is a select statement:

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

We’ve aliased the max(report_run_date) as maxdate and we’ve aliased the entire result set as maxresults.

Now, that we’ve created this select statement within our FROM clause, Oracle will let us join these results against our original report_history table. So we’ve joined the report_name and report_run_date fields between the tables called rh and maxresults. This allows us to retrieve the report_name, max(report_run_date) as well as the user_name.

Question: I need help with a SQL query. I have a table in Oracle called orders which has the following fields: order_no, customer, and amount.

I need a query that will return the customer who has ordered the highest total amount.

Answer: The following SQL should return the customer with the highest total amount in the orders table.

SELECT query1.*
FROM (SELECT customer, SUM(orders.amount) AS total_amt
      FROM orders
      GROUP BY orders.customer) query1,

     (SELECT MAX(query2.total_amt) AS highest_amt
      FROM (SELECT customer, SUM(orders.amount) AS total_amt
            FROM orders
            GROUP BY orders.customer) query2) query3
WHERE query1.total_amt = query3.highest_amt;

This SQL SELECT statement will summarize the total orders for each customer and then return the customer with the highest total orders. This syntax is optimized for Oracle and may not work for other database technologies.

Question: I’m trying to retrieve some info from an Oracle database. I’ve got a table named Scoring with two fields — Name and Score. What I want to get is the highest score from the table and the name of the player.

Answer: The following SQL SELECT statement should work:

SELECT Name, Score
FROM Scoring
WHERE Score = (SELECT MAX(Score) FROM Scoring);

Question: I need help in a SQL query. I have a table in Oracle called cust_order which has the following fields: OrderNo, Customer_id, Order_Date, and Amount.

I would like to find the customer_id, who has Highest order count.

I tried with following query.

SELECT MAX(COUNT(*))
FROM CUST_ORDER
GROUP BY CUSTOMER_ID;

This gives me the max Count, But, I can’t get the CUSTOMER_ID. Can you help me please?

Answer: The following SQL SELECT statement should return the customer with the highest order count in the cust_order table.

SELECT query1.*
FROM (SELECT Customer_id, Count(*) AS order_count
      FROM cust_order
      GROUP BY cust_order.Customer_id) query1,

     (SELECT max(query2.order_count) AS highest_count
      FROM (SELECT Customer_id, Count(*) AS order_count
            FROM cust_order
            GROUP BY cust_order.Customer_id) query2) query3
WHERE query1.order_count = query3.highest_count;

This SQL SELECT statement will summarize the total orders for each customer and then return the customer with the highest order count. This syntax is optimized for Oracle and may not work for other database technologies.

Question: I’m trying to get the employee with the maximum salary from department 30, but I need to display the employee’s full information. I’ve tried the following query, but it returns the result from both department 30 and 80:

SELECT *
FROM employees
WHERE salary = (SELECT MAX(salary)
                FROM employees
                WHERE department_id=30);

Answer: The SQL SELECT statement that you have written will first determine the maximum salary for department 30, but then you select all employees that have this salary. In your case, you must have 2 employees (one in department 30 and another in department 80) that have this same salary. You need to make sure that you are refining your query results to only return employees from department 30.

Try using this SQL SELECT statement:

SELECT *
FROM employees
WHERE department_id=30
AND salary = (SELECT MAX(salary)
              FROM employees
              WHERE department_id=30);

This will return the employee information for only the employee in department 30 that has the highest salary.

Источник

В этом учебном материале вы узнаете, как использовать SQL функцию MAX с синтаксисом примерами.

Описание

SQL функция MAX используется для возврата максимального значения выражения в операторе SELECT.

Синтаксис

Синтаксис для функции MAX в SQL.

SELECT MAX(aggregate_expression)
FROM tables
[WHERE conditions];

Или синтаксис для функции MAX при группировке результатов по одному или нескольким столбцам.

SELECT expression1, expression2, … expression_n,
MAX(aggregate_expression)
FROM tables
[WHERE conditions]
GROUP BY expression1, expression2, … expression_n;

Параметры или аргумент

expression1, expression2, … expression_n: Выражения, которые не инкапсулированы в функцию MAX и должны быть включены в предложение GROUP BY в конце SQL запроса
aggregate_expression: Это столбец или выражение, из которого будет возвращено максимальное значение
tables: Таблицы, из которых вы хотите получить записи. В предложении FROM должна быть указана хотя бы одна таблица
WHERE conditions: Необязательный. Это условия, которые должны быть выполнены для выбора записей

Пример — с одним выражение

Простейшим способом использования SQL функции MAX будет возвращение одного поля, которое вычисляет значение MAX.
Например, вы можете узнать максимальную зарплату всех сотрудников.

SELECT MAX(salary) AS «Highest salary»

FROM employees;

В этом примере SQL функции MAX мы выражению MAX(salary) указали псевдоним «Highest salary». В результате «Highest salary» будет отображаться как имя поля при возврате набора результатов.

Пример — использование SQL оператора GROUP BY

В некоторых случаях вам потребуется использовать SQL оператор GROUP BY с функцией SQL MAX.
Например, вы также можете использовать SQL функцию MAX, чтобы вернуть название каждого отдела и максимальную зарплату в отделе.

SELECT department, MAX(salary) AS «Highest salary»

FROM employees

GROUP BY department;

Поскольку в SQL операторе SELECT вы указали один столбец, который не инкапсулирован в функции MAX, вы должны использовать SQL оператор GROUP BY. Поэтому поле department должно быть указано в разделе GROUP BY.

Источник

Оператор SQL MAX() — функция возвращающая максимальное значение столбца таблицы.

Функция SQL MAX() имеет следующий синтаксис:

MAX(column_name)

Примеры оператора SQL MAX: Имеется следующая таблица Universities:

ID	UniversityName	Students	Faculties	Professores	Location	Site
1	Perm State National Research University	12400	12	1229	Perm	psu.ru
2	Saint Petersburg State University	21300	24	13126	Saint-Petersburg	spbu.ru
3	Novosibirsk State University	7200	13	1527	Novosibirsk	nsu.ru
4	Moscow State University	35100	39	14358	Moscow	msu.ru
5	Higher School of Economics	20335	12	1615	Moscow	hse.ru
6	Ural Federal University	57000	19	5640	Yekaterinburg	urfu.ru
7	National Research Nuclear University	8600	10	936	Moscow	mephi.ru

Пример 1: Используя оператор SQL MAX найти максимальное значение колонки Students:

SELECT MAX(Students)
FROM Univerities

Ответ: 57000

Пример 2: Используя оператор SQL MAX найти максимальное значение колонки Faculties, где расположение университета (Location) — Saint-Petersburg:

SELECT MAX(Faculties)
FROM Universities
WHERE Location = 'Saint-Petersburg'

Ответ: 24

Пример 3: Используя оператор SQL MAX найти университет (UniversityName) с наибольшим числом преподавателей (Professores):

SELECT UniversityName
FROM University
WHERE Professores = (SELECT MAX(Professores) FROM Universities)

Ответ: Moscow State University

Источник

For example:

name | weight
jon    100    
jane   120    
joe    130

How do I only return the name of the person with the largest weight?

peterh

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asked Feb 6, 2011 at 0:54

SELECT name FROM tbl ORDER BY weight DESC LIMIT 1

Much more performant than the other answer and results in one row only.

answered Dec 5, 2012 at 11:20

raverenraveren

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Use this:

select name
from tbl
where weight = (select max(weight) from tbl)

answered Feb 6, 2011 at 0:58

HaoHao

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If you need to find multiple rows, e.g. date on which each person had maximum weight:

name | weight | day
don    110      1
don    120      20
don    110      30
joe    90       1
joe    80       15
joe    85       30

i.e. for «don» you want to get "don | 120 | 20" and for joe you want "joe | 90 | 1", then you can write:

SELECT name, max(weight), (array_agg(day ORDER BY weight DESC))[1] FROM tbl GROUP BY name

answered Nov 3, 2018 at 16:30

alexkovelskyalexkovelsky

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Источник