100 important knowledge points that SQL must master: sorting and retrieving data

3.1 Sorting data
As mentioned in the previous lesson , Below SQL Statement returns a single column of a database table . But look at
Its output , There is no specific order

Input ▼
SELECT prod_name
FROM Products;

Output ▼ 
prod_name
--------------------
Fish bean bag toy
Bird bean bag toy
Rabbit bean bag toy
8 inch teddy bear
12 inch teddy bear
18 inch teddy bear
Raggedy Ann
King doll
Queen doll

Actually , The retrieved data is not displayed randomly . If you don't sort , Data will generally be represented by it
The order in which they appear in the table shows , This may be the order in which the data was first added to the table . however ,
If the data is subsequently updated or deleted , Then this order will be affected by DBMS Reuse back
The impact of the way storage space is collected . therefore , If there's no clear control , Then the final result
You can't （ Should not be ） Depending on the sort order . Relational database design theory holds that , If you don't
Specify the sort order , The order of retrieved data should not be assumed to have any meaning .

Clause （clause）
SQL A sentence is made up of clauses , Some clauses are necessary , Some are optional . A dime
A sentence usually consists of a keyword plus the data provided . The example of clause is as follows
What I saw in the first lesson SELECT Of the statement FROM Clause .

In order to make clear the order SELECT The data retrieved by the statement , You can use ORDER BY Clause .
ORDER BY Clause takes the name of one or more columns , Sort the output accordingly . Please see the following
Example ：
Input ▼
SELECT prod_name
FROM Products
ORDER BY prod_name;
analysis ▼
Except for instructions DBMS Software pair prod_name Columns sort data in alphabetical order ORDER BY
Out of clause , This statement is the same as the previous statement . give the result as follows .
Output ▼
prod_name
--------------------
12 inch teddy bear
18 inch teddy bear
8 inch teddy bear
Bird bean bag toy
Fish bean bag toy
King doll
Queen doll
Rabbit bean bag toy
Raggedy Ann 

Be careful ： ORDER BY Position of clause
In the specified section ORDER BY When clause , It should be guaranteed that it is SELECT The last word in the sentence
Tiaozi sentence . If it's not the last clause , Will go wrong .

Tips ： Sort by non selected columns
Usually , ORDER BY The columns used in the clause will be the columns selected for display . however , real
This is not necessarily the case , Non - Legal columns are used to retrieve non - legal data . 

3.2 Sort by multiple columns
It is often necessary to sort data by more than one column . for example , If you want to display the employee list , can
Can you wish to sort by last name and first name （ First, sort by surname , Then sort by first name in each surname ）. Such as
If more than one employee has the same last name , This is very useful .
To sort by multiple columns , Just specify these column names , Separate column names with commas （ It's like
When multiple columns are selected ）.
The following code searches 3 Columns , And sort the results by two columns —— First, according to the price
grid , Then sort by name .
Input ▼
SELECT prod_id, prod_price, prod_name
FROM Products
ORDER BY prod_price, prod_name; 

Output ▼ 

prod_id prod_price prod_name
------- ---------- --------------------
BNBG02     3.4900     Bird bean bag toy
BNBG01     3.4900     Fish bean bag toy
BNBG03     3.4900     Rabbit bean bag toy
RGAN01     4.9900     Raggedy Ann
BR01       5.9900     8 inch teddy bear
BR02       8.9900     12 inch teddy bear
RYL01      9.4900     King doll
RYL02      9.4900     Queen doll
BR03       11.9900    18 inch teddy bear

It is important to understand that when sorting by multiple columns , The order of sorting is completely in accordance with the regulations . In other words,
say , For the output in the above example , Only if more than one row has the same prod_price When the value of
Just press prod_name Sort . If prod_price All values in the column are
Unique , You don't press prod_name Sort .
3.3 Sort by column position
In addition to being able to indicate the sort order by column name , ORDER BY It also supports sorting by relative column position
order . In order to understand this content , Let's take an example ：
Input ▼
SELECT prod_id, prod_price, prod_name
FROM Products
ORDER BY 2, 3;
Output ▼ 

prod_id prod_price prod_name
------- ---------- --------------------
BNBG02     3.4900     Bird bean bag toy
BNBG01     3.4900     Fish bean bag toy
BNBG03     3.4900     Rabbit bean bag toy
RGAN01     4.9900     Raggedy Ann
BR01       5.9900     8 inch teddy bear
BR02       8.9900     12 inch teddy bear
RYL01      9.4900     King doll
RYL02      9.4900     Queen doll
BR03       11.9900    18 inch teddy bear

analysis ▼ 
You can see , The output here is the same as the query above , The difference is ORDER BY Son
sentence . SELECT What is specified in the listing is the relative position of the selected column, not the column name . ORDER BY 2
According to the said SELECT The second column in the list prod_price Sort . ORDER BY 2,
3 First press prod_price , Press again prod_name Sort .
The main benefit of this technique is that you don't have to re-enter the column name . But it also has disadvantages . First , No
Giving the column name explicitly may cause misusing the column name to sort . secondly , In the face of SELECT The list is updated
It is easy to sort the data incorrectly （ Forget about ORDER BY Change the clause accordingly ）.
Last , If the column to be sorted is not in SELECT In the list , Obviously you can't use this technology .
Tips ： Sort by unselected column
obviously , When the basis does not appear in SELECT When sorting columns in the list , This... Cannot be used
technology . however , If necessary , You can mix actual column names with relative column positions .
3.4 Specify sorting direction
Data sorting is not limited to ascending sorting （ from A To Z）, This is the default sort order . just so so
Use ORDER BY Clauses in descending order （ from Z To A） Sort . For descending sort ,
Must specify DESC keyword .
The following example sorts products in descending order of price （ The most expensive is at the top ）：

Input ▼
SELECT prod_id, prod_price, prod_name
FROM Products
ORDER BY prod_price DESC;
Output ▼

If you plan to sort with multiple columns , What should I do ？ Sort the products in descending order （ Most expensive
Of is at the front ）, Plus the product name ：
Input ▼
SELECT prod_id, prod_price, prod_name
FROM Products
ORDER BY prod_price DESC, prod_name;
Output ▼

prod_id prod_price prod_name
------- ---------- --------------------
BR03       11.9900    18 inch teddy bear
RYL01      9.4900     King doll
RYL02      9.4900     Queen doll
BR02       8.9900     12 inch teddy bear
BR01       5.9900     8 inch teddy bear
RGAN01     4.9900     Raggedy Ann
BNBG02     3.4900     Bird bean bag toy
BNBG01     3.4900     Fish bean bag toy
BNBG03     3.4900     Rabbit bean bag toy

  analysis ▼
DESC Keywords only apply to column names directly before them . In the example above , Only right prod_price
Column specifies DESC , Yes prod_name Column does not specify . therefore , prod_price Columns are arranged in descending order
order , and prod_name Column （ Within each price ） Still sort in standard ascending order .

Warning ： Sort in descending order on multiple columns
If you want to sort in descending order on multiple columns , You must specify... For each column DESC keyword .

Please note that , DESC yes DESCENDING Abbreviation , Both of these keywords can be used . And DESC
The opposite is ASC （ or ASCENDING ）, You can specify it when sorting in ascending order . But actually ,
ASC It's not much use , Because ascending is the default （ If you don't specify ASC It doesn't specify
DESC , Then assume ASC ）.

Tips ： Case sensitive and sort order
When sorting textual data , A And a Is it the same ？ a be located B Before , still Z
after ？ These are not theoretical issues , The answer depends on how the database is set up .
In dictionary （dictionary） In the sort order , A Be regarded as being associated with a identical , This is most numbers
According to the default practice of the library management system . however , many DBMS Allow database administrators to
It's time to change this behavior （ If your database contains a large number of foreign language characters , It may have to be
Sample preparation ）.
The key question here is , If you really need to change this sort order , With simple ORDER
BY Clause may not be able to do so . You must ask the database administrator for help .