100 important knowledge points that SQL must master: summary data

9.1 Aggregation function
We often need to aggregate data without actually retrieving it , So SQL Provides a dedicated
Functions of doors . Use these functions ,SQL Queries can be used to retrieve data , For analysis and reporting
Generate . Examples of this type of retrieval are ：
   Determine the number of rows in the table （ Or the number of rows that satisfy a condition or contain a specific value ）;
   Get the sum of some rows in the table ;
   Find table columns （ Or all or some particular line ） The maximum of 、 minimum value 、 Average .
All the above examples need to summarize the data in the table , Without having to find out the data itself . therefore , return
Returning the actual table data is a waste of time and processing resources （ Not to mention bandwidth ）. Say it again ,
What we really want is to summarize information .
To facilitate this type of Retrieval ,SQL given 5 Aggregate functions , See table 9-1. These functions
Can carry out the above search . Different from the data processing functions introduced in the previous chapter ,SQL The aggregation function of
In all kinds of major SQL There is quite consistent support in the implementation .

Aggregation function （aggregate function）
Functions that run on certain lines , Calculates and returns a value .
surface 9-1 SQL Aggregation function
Letter Count   say bright
AVG()   Returns the average of a column
COUNT()   Returns the number of rows in a column
MAX()   Returns the maximum value of a column
MIN()   Returns the minimum value of a column
SUM()   Returns the sum of values in a column
The following describes the use of each function .
9.1.1  AVG() function
AVG() By counting the number of rows in the table and calculating the sum of their column values , Find the average value of this column . AVG()
Can be used to return the average of all columns , It can also be used to return the average value of a specific column or row .
The following example uses AVG() return Products The average price of all the products in the table ：
Input ▼
SELECT AVG(prod_price) AS avg_price
FROM Products;
Output ▼
avg_price
-------------
6.823333
analysis ▼
this SELECT Statement return value avg_price , It contains Products For all products in the table
The average price . As the first 7 Lesson description , avg_price It's an alias .

AVG() It can also be used to determine the average value of a specific column or row . The following example returns a specific supply
The average price of a product offered by a supplier ：
Input ▼
SELECT AVG(prod_price) AS avg_price
FROM Products
WHERE vend_id = 'DLL01';
Output ▼
avg_price
-----------
3.8650
analysis ▼
This article SELECT The difference between the statement and the previous one is that , It contains WHERE Clause . this
WHERE Clause only filters out vend_id by DLL01 Products , therefore avg_price Middle return
The returned value is only the average value of the supplier's products .
Be careful ： Only for a single column
AVG() It can only be used to determine the average of a particular numeric column , And the column name must be a function parameter
The number gives . To get the average of multiple columns , You have to use multiple AVG() function . Only
An exception is when you want to calculate a value from multiple columns , Later in this lesson, we will talk about .
explain ： NULL value
AVG() Function ignores column values as NULL The line of .
9.1.2  COUNT() function
COUNT() Function to count . available COUNT() Determine the number of rows in a table or match a specific
Number of conditional rows .

COUNT() Functions can be used in two ways ：
   Use COUNT(*) Count the number of rows in the table , Whether a table column contains null values
（ NULL ） Or non null .
   Use COUNT(column) Count rows with values in a specific column , Ignore NULL value .
The following example returns Customers The total number of customers in the table ：
Input ▼
SELECT COUNT(*) AS num_cust
FROM Customers;
Output ▼
num_cust
--------
5
analysis ▼
In this example , utilize COUNT(*) Count all rows , Whatever the value of each column in the row . meter
Values in num_cust Back in .
The following example counts only customers with email addresses ：
Input ▼
SELECT COUNT(cust_email) AS num_cust
FROM Customers;
Output ▼
num_cust
--------
3

analysis ▼
This article SELECT Statements use COUNT(cust_email) Yes cust_email Columns with values
Count rows . In this example , cust_email Count as 3 （ Express 5 Only... Of the customers
Yes 3 The customer has an email address ）.
explain ： NULL value
If you specify a column name , be COUNT() The function ignores that the value of the specified column is NULL The line of , but
If COUNT() The asterisk is used in the function （ * ）, Don't ignore .
9.1.3  MAX() function
MAX() Returns the maximum value in the specified column . MAX() Require column name , As shown below ：
Input ▼
SELECT MAX(prod_price) AS max_price
FROM Products;
Output ▼
max_price
----------
11.9900
analysis ▼
here , MAX() return Products The price of the most expensive item in the list .
Tips ： Use... For non numerical data MAX()
although MAX() Usually used to find the maximum value or date value , But many （ Not all ）
DBMS Allows it to be used to return the maximum value in any column , Include the most... In the returned text column
Great value . When used for text data , MAX() Returns the last row sorted by this column .

explain ： NULL value
MAX() Function ignores column values as NULL The line of .
9.1.4  MIN() function
MIN() The function of is just the same as MAX() The function is the opposite , It returns the minimum value of the specified column . And MAX()
equally , MIN() Require column name , As shown below ：
Input ▼
SELECT MIN(prod_price) AS min_price
FROM Products;
Output ▼
min_price
----------
3.4900
analysis ▼
among MIN() return Products The price of the cheapest item in the list .
Tips ： Use... For non numerical data MIN()
although MIN() Usually used to find the minimum value or date value , But many （ Not all ）
DBMS Allows it to be used to return the minimum value in any column , Include the most... In the returned text column
Small value . When used for text data , MIN() Returns the first row after the column is sorted .
explain ： NULL value
MIN() Function ignores column values as NULL The line of .
9.1.5  SUM() function
SUM() Used to return the sum of the specified column values （ A total of ）.

Here's an example , OrderItems Include the actual items in the order , Each item has a corresponding
The number of . The total number of items ordered can be retrieved as follows （ all quantity Sum of values ）：
Input ▼
SELECT SUM(quantity) AS items_ordered
FROM OrderItems
WHERE order_num = 20005;
Output ▼
items_ordered
----------
200
analysis ▼
function SUM(quantity) Return the sum of the quantity of all items in the order , WHERE Clause guarantees only
Count the items in an item order .
SUM() It can also be used to sum up the calculated value . In the following example , Total... For each item
item_price*quantity , Get the total order amount ：
Input ▼
SELECT SUM(item_price*quantity) AS total_price
FROM OrderItems
WHERE order_num = 20005;
Output ▼
total_price
----------
1648.0000
analysis ▼
function SUM(item_price*quantity) Return the sum of the prices of all items in the order , WHERE

Clause also ensures that only the items in an item order are counted .
Tips ： Calculate on multiple columns
As shown in this example , Using standard arithmetic operators , All aggregate functions can be used to perform multiple
Calculation on Columns .
explain ： NULL value
SUM() Function ignores column values as NULL The line of .
9.2 Gather different values
above 5 Each aggregation function can be used as follows .
   Perform calculations on all lines , Appoint ALL Parameters or do not specify parameters （ because ALL Is the default line
by ）.
   Contains only different values , Appoint DISTINCT Parameters .
Tips ： ALL As the default
ALL Parameters do not need to be specified , Because it's the default behavior . If you don't specify DISTINCT , be
Is assumed to be ALL .
The following example uses AVG() Function returns the average price of products provided by a specific supplier . it
With the above SELECT Same statement , But using the DISTINCT Parameters , So the average value is only
Consider different prices ：
Input ▼
SELECT AVG(DISTINCT prod_price) AS avg_price
FROM Products
WHERE vend_id = 'DLL01';

Output ▼
avg_price
-----------
4.2400
analysis ▼
You can see , In the use of the DISTINCT after , In this example avg_price Relatively high , because
For multiple items with the same lower price . Excluding them raises the average price .
Be careful ： DISTINCT Cannot be used for COUNT(*)
If you specify a column name , be DISTINCT It can only be used for COUNT() . DISTINCT Out-of-service
On COUNT(*) . Similarly , DISTINCT Column names must be used , Cannot be used for calculations or tables
Da da .
Tips ： take DISTINCT be used for MIN() and MAX()
although DISTINCT Technically, it can be used for MIN() and MAX() , But it doesn't actually
valuable . The minimum and maximum values in a column regardless of whether only different values are considered , The results are
It's the same .
explain ： Other aggregation parameters
Except for what's introduced here DISTINCT and ALL Parameters , yes , we have DBMS Other parameters are also supported ,
For example, it supports the calculation of subsets of query results TOP and TOP PERCENT . To solve the problem
Somatic DBMS Which parameters are supported , Please refer to the corresponding documentation .
9.3 Combining aggregate functions
All the aggregation function examples so far only involve a single function . But actually , SELECT language
Sentences can contain multiple aggregate functions as needed . Please see the following example ：

Input ▼
SELECT COUNT(*) AS num_items,
MIN(prod_price) AS price_min,
MAX(prod_price) AS price_max,
AVG(prod_price) AS price_avg
FROM Products;
Output ▼
num_items price_min price_max price_avg
---------- --------------- --------------- ---------
9 3.4900 11.9900 6.823333
analysis ▼
Here's a single SELECT Statement executed 4 Aggregate calculation , return 4 It's worth （ Products
The number of items in the table , The highest value of product price 、 Minimum and average ）.
Be careful ： Take the alias
When specifying an alias to include the result of an aggregate function , The actual columns in the table should not be used
name . Although it's legal to do so , But many SQL Implementation does not support , Blurring may occur
Error messages for .