100 important knowledge points that SQL must master: creating calculation fields

7.1 Calculated field
The data stored in the database table is generally not in the format required by the application , Here are some examples
Example .
   The company name needs to be displayed , You also need to display the address of the company , But these two messages are stored in
In different table columns .
   City 、 The state and zip code are stored in different columns （ It should be ）, But the e-mail label is marked
The printer needs to retrieve them as a properly formatted field .
   Column data is case mixed , But the report program needs to show all data in uppercase .
   The item order table stores the price and quantity of the item , Don't store the total price of each item （ Price
Multiply the lattice by the quantity ）. But for printing invoices , The total price of the goods needed .
   It is necessary to make statistics according to the table data, such as total 、 Calculation of the average .
In each of the above examples , None of the data stored in the table is required by the application . We
You need to retrieve the transformation directly from the database 、 Calculated or formatted data , Instead of retrieving
Data , Then reformat it in the client application .

This is where calculated fields can come in handy . Different from the columns introduced in previous lessons , Calculation
The field does not actually exist in the database table . The calculation field is run at SELECT Statement within
Created .
Field （field）
Basically with columns （column） It means the same , Often used interchangeably , But database column one
Generally called column , The term field is usually used in the case of calculating fields .
Special attention required , Only the database knows SELECT Which columns in the statement are the actual table columns ,
Which columns are calculated fields . From the client （ Like an application ） Look at , Calculate the data of the field and
The data of other columns is returned in the same way .
Tips ： Format of client and server
stay SQL Many of the transformations and formatting that can be done within a statement can be done directly on the client side
Within the application . But generally speaking , It is better to complete these operations on the database server than in
It is much faster to complete in the client .
7.2 Splicing field
To illustrate how to use calculated fields , Let's take a simple example , Create two columns
The title of the .
Vendors The table contains supplier name and address information . If you want to generate a supplier report , Need to be
To format the name （ Location ） List the supplier's location in .
This report requires a value , The data in the table is stored in two columns vend_name and vend_country
in . Besides , You need to use parentheses vend_country Cover up , These things are not stored
In the database table . This returns the name and address of the supplier SELECT The sentence is very simple , but
How do we create this composite value ？

Splicing （concatenate）
Join values together （ Append one value to another ） Form a single value .
The solution is to splice the two columns . stay SQL Medium SELECT In the sentence , One can be used
A special operator to splice two columns . According to what you use DBMS, This operator can be used
plus （ + ） Or two vertical bars （ || ） Express . stay MySQL and MariaDB in , You have to use
Special functions .
explain ： yes + still || ？
SQL Server Use + Number .DB2、Oracle、PostgreSQL and SQLite Use || . detailed
Please refer to the specific DBMS file .
Here is an example of using the plus sign （ Most of the DBMS Use this grammar ）：
Input ▼
SELECT vend_name + '(' + vend_country + ')'
FROM Vendors
ORDER BY vend_name;
Output ▼
-----------------------------------------------------------
Bear Emporium (USA )
Bears R Us (USA )
Doll House Inc. (USA )
Fun and Games (England )
Furball Inc. (USA )
Jouets et ours (France )
The following is the same statement , But it uses || grammar ：
Input ▼
SELECT vend_name || '(' || vend_country || ')'

FROM Vendors
ORDER BY vend_name;
Output ▼
-----------------------------------------------------------
Bear Emporium (USA )
Bears R Us (USA )
Doll House Inc. (USA )
Fun and Games (England )
Furball Inc. (USA )
Jouets et ours (France )
Here's how to use MySQL or MariaDB The statements you need to use ：
Input ▼
SELECT Concat(vend_name, ' (', vend_country, ')')
FROM Vendors
ORDER BY vend_name;
analysis ▼
The top two SELECT Statement to splice the following elements ：
   Stored in vend_name The names in the columns ;
   A string containing a space and an open parenthesis ;
   Stored in vend_country The countries in the list ;
   A string containing a closing parenthesis .
From the above output, you can see , SELECT Statement returns a column containing the above four elements
（ Calculated field ）.
Look at the above SELECT The output returned by the statement . Two columns combined into one calculation field are used
Fill in the blanks . Many databases （ Not all ） Save text values filled with column widths , But actually
You don't need these spaces for the results you want . To return formatted data correctly , This must be removed

Some spaces . This can be used SQL Of RTRIM() Function to complete , As shown below ：
Input ▼
SELECT RTRIM(vend_name) + ' (' + RTRIM(vend_country) + ')'
FROM Vendors
ORDER BY vend_name;
Output ▼
-----------------------------------------------------------
Bear Emporium (USA)
Bears R Us (USA)
Doll House Inc. (USA)
Fun and Games (England)
Furball Inc. (USA)
Jouets et ours (France)
The following is the same statement , But it uses || ：
Input ▼
SELECT RTRIM(vend_name) || ' (' || RTRIM(vend_country) || ')'
FROM Vendors
ORDER BY vend_name;
Output ▼
-----------------------------------------------------------
Bear Emporium (USA)
Bears R Us (USA)
Doll House Inc. (USA)
Fun and Games (England)
Furball Inc. (USA)
Jouets et ours (France)
analysis ▼
RTRIM() Function to remove all spaces to the right of the value . By using RTRIM() , Each column goes into
All right, tidy up .

explain ：TRIM function
majority DBMS All support RTRIM() （ As I just saw , It removes the right side of the string
Space ）、 LTRIM() （ Remove the space to the left of the string ） as well as TRIM() （ Remove the characters
The spaces on the left and right of the string ）.
Use the alias
As you can see from the previous output , SELECT Statement can splice address fields very well . however ,
What is the name of this new calculation column ？ It doesn't actually have a name , It's just a value . Such as
Only in SQL Check the results in the query tool , There's nothing wrong with that . however , One
Unnamed columns cannot be used in client applications , Because the client has no way to reference it .
To solve this problem ,SQL Support column aliases . Alias （alias） Is a field or value
Alternate name . It's called AS Key words are given to . Please look at the following SELECT sentence ：
Input ▼
SELECT RTRIM(vend_name) + ' (' + RTRIM(vend_country) + ')'
AS vend_title
FROM Vendors
ORDER BY vend_name;
Output ▼
vend_title
-----------------------------------------------------------
Bear Emporium (USA)
Bears R Us (USA)
Doll House Inc. (USA)
Fun and Games (England)
Furball Inc. (USA)
Jouets et ours (France)
The following is the same statement , But it uses || grammar ： 

Input ▼
SELECT RTRIM(vend_name) || ' (' || RTRIM(vend_country) || ')'
AS vend_title
FROM Vendors
ORDER BY vend_name;
Here is MySQL and MariaDB Statements used in ： 
Input ▼
SELECT Concat(RTrim(vend_name), ' (',
RTrim(vend_country), ')') AS vend_title
FROM Vendors
ORDER BY vend_name;
analysis ▼
SELECT The statement itself is the same as previously used , Only the calculation field here is followed by the text
Ben AS vend_title . It indicates SQL Create a named... That contains the specified calculation result
vend_title Calculation field of . You can see from the output , The result is the same as before , But now
In the column named vend_title , Any client application can reference this column by name , It's like
It is an actual table column like .
explain ：AS Usually optional
In many DBMS in , AS Keywords are optional , But it's best to use it , This is considered a
Best practices .
Tips ： Other uses of aliases
Aliases have other uses . Common uses include the inclusion of illegal in actual table column names
character （ Such as space ） Rename it when , When the original name is ambiguous or easily misunderstood
Charge it .

Be careful ： Alias
The alias name can be a word , It can also be a string . If it's the latter ,
String should be enclosed in quotation marks . Although this practice is legal , But it's not recommended .
Multi word names are highly readable , However, it will bring various problems to the client application . therefore ,
The most common use of aliases is to rename the column name of multiple words to the name of one word .
explain ： Export column
Aliases are sometimes referred to as export columns （derived column）, Anyway , They represent
Is the same thing .
7.3 Perform arithmetic calculations
Another common use of calculation fields is to perform arithmetic calculations on the retrieved data . for instance ,
Orders The table contains all orders received , OrderItems The table contains the items in each order
goods . Below SQL Statement to retrieve the order number 20008 All of the items in ：
Input ▼
SELECT prod_id, quantity, item_price
FROM OrderItems
WHERE order_num = 20008;
Output ▼
prod_id quantity item_price
---------- ----------- ---------------------
RGAN01 5 4.9900
BR03 5 11.9900
BNBG01 10 3.4900
BNBG02 10 3.4900
BNBG03 10 3.4900
item_price The column contains the unit price of each item in the order . The prices of the items are summarized below （ Unit price times order quantity ）：
Input ▼
SELECT prod_id,
quantity,
item_price,
quantity*item_price AS expanded_price
FROM OrderItems
WHERE order_num = 20008;
Output ▼
prod_id quantity item_price expanded_price
---------- ----------- ------------ -----------------
RGAN01 5 4.9900 24.9500
BR03 5 11.9900 59.9500
BNBG01 10 3.4900 34.9000
BNBG02 10 3.4900 34.9000
BNBG03 10 3.4900 34.9000
analysis ▼
The... Shown in the output expanded_price A column is a calculated field , This calculation is quantity*
item_price . Client applications can now use this new computed column , Just like using other columns
equally .
SQL Support table 7-1 The basic arithmetic operators listed in . Besides , Parentheses can be used to distinguish between good and bad
Order first . Introduction to priorities , Please refer to the first 5 course .
surface 7-1 SQL arithmetic operator
fuck do operator   say bright
+         Add
-         reduce
*         ride
/         except

Tips ： How to test and calculate
SELECT The statement is test 、 Test functions and calculations provide a good way . although SELECT
Usually used to retrieve data from tables , But omitted FROM Clause is followed by a simple access and
Processing expressions , for example SELECT 3 * 2; Will return 6 , SELECT Trim(' abc ');
Will return abc , SELECT Curdate(); Use Curdate() Function returns the current date
And time . Now you see , It can be used as needed SELECT Statement to verify .