MySQL row to column, column to row, multiple columns to one row, one row to multiple columns

This blog is about the interview I saw before SQL topic Used it to learn After reading this blog , I believe you will be right SQL Grammar has different feelings and understandings .

Version Description

mysql8.0

One 、 Transfer line column

demand

classic case when Realization

select

name,

max(case subject when ' Chinese language and literature ' then resuilt else 0 end) ' Chinese language and literature ',

max(case subject when ' mathematics ' then resuilt else 0 end) ' mathematics ',

max(case subject when ' Physics ' then resuilt else 0 end) ' Physics '

from student group by name

Use if sentence

Similar in general case when

select name,

max(if(subject = ' Chinese language and literature ',resuilt,0)) ' Chinese language and literature '

from student group by name

Dynamic splicing sql sentence , No matter how many rows are transferred

set @sql = null;

select group_concat(distinct concat('max(if(a.subject = ''',a.subject,''', a.resuilt, 0)) as ''',a.subject, '''')) into @sql from student a;

set @sql = concat('select name,', @sql, 'from student a group by a.name' );

prepare stmt from @sql; -- Dynamically generate scripts , Prepare a statement

execute stmt; -- Executing scripts dynamically , Execute the prepared statement

deallocate prepare stmt; -- Release prepared statements

Extended analysis

mysql Why do I have to use sum perhaps max

In case of this problem, we can debug sql, To solve our doubts .

Two 、 Column turned

demand

take

Convert to

select
name,
' Chinese language and literature ' as subject,
chinese as resuilt
from -- Note that the normal here should be from Directly followed by a table Lazy don't want to create a table structure Direct subquery ( Row to column code )
(select
name,
max(case subject when ' Chinese language and literature ' then resuilt else 0 end) 'chinese',
max(case subject when ' mathematics ' then resuilt else 0 end) 'math',
max(case subject when ' Physics ' then resuilt else 0 end) 'physics'
from student group by name
) t

union all

select
name,
' mathematics ' as subject,
math as resuilt
from -- Note that the normal here should be from Directly followed by a table Lazy don't want to create a table structure Direct subquery ( Row to column code )
(select
name,
max(case subject when ' Chinese language and literature ' then resuilt else 0 end) 'chinese',
max(case subject when ' mathematics ' then resuilt else 0 end) 'math',
max(case subject when ' Physics ' then resuilt else 0 end) 'physics'
from student group by name
) t

union all

select
name,
' Physics ' as subject,
physics as resuilt
from -- Note that the normal here should be from Directly followed by a table Lazy don't want to create a table structure Direct subquery ( Row to column code )
(select
name,
max(case subject when ' Chinese language and literature ' then resuilt else 0 end) 'chinese',
max(case subject when ' mathematics ' then resuilt else 0 end) 'math',
max(case subject when ' Physics ' then resuilt else 0 end) 'physics'
from student group by name
) t

 

summary

Transfer line column , Use case…when Query data by situation ,group by and sum/max Screening

Column turned , Use... For each column of data required for query union perhaps union all Union

3、 ... and 、 Multiple columns to one row

demand

take

Turn into

---

select name,GROUP_CONCAT(`subject`,':',resuilt) ' achievement ' from student group by name

GROUP_CONCAT(expr) This function combines the non empty column values according to the grouping conditions and finally returns . If there is a null value , Then return to empty

Four 、 One row to many columns

demand

take

Turn into

 a surface

select 
name,
marks, 
case 
 when locate(' Chinese language and literature ',marks) > 0 then substring_index(substring_index(marks,' Chinese language and literature :',-1),',',1) else 0 end 
as  Chinese language and literature  ,
case 
 when locate(' mathematics ',marks) > 0 then substring_index(substring_index(marks,' mathematics :',-1),',',1) else 0 end 
as  mathematics  ,
case 
 when locate(' Physics ',marks) > 0 then substring_index(substring_index(marks,' Physics :',-1),',',1) else 0 end 
as  Physics  ,
case 
 when locate(' history ',marks) > 0 then substring_index(substring_index(marks,' history :',-1),',',1) else 0 end 
as  history  
from 
(select
name,
GROUP_CONCAT(subject,':',resuilt) marks
from
student group by name) a

It's just the first step give an example The request is still one step away Use union all Continue processing sql

b surface

select
name,
' Chinese language and literature ' subject,
 Chinese language and literature  score
from 
(a surface ) b

union all

select
name,
' mathematics ' subject,
 mathematics  score
from 
(a surface ) b

union all

select
name,
' Physics ' subject,
 Physics  score
from 
(a surface ) b

union all

select
name,
' history ' subject,
 history  score
from 
(a surface ) b

Locate The main function is to determine whether a string contains another string , Such as

Locate(str,sub) > 0, Express sub The string contains str character string ;

Locate(str,sub) = 0, Express sub String does not contain str character string .

substring_index( Parameters str, Parameters delim, Parameters count)

str : What to deal with character string

delim: Separator

count: Count

in other words , If count Positive number , So it's counting from left to right , The first N All of the contents to the left of the separator ! contrary , If it's a negative number , So it's counting from the right , The first N Everything to the right of the separator ,

Reference article

mysql Transfer line column , Column turned , And why it is used when transferring rows to columns max function _ The blog of a daffodil swaying in the wind -CSDN Blog _max function Transfer line column

elementary analysis MySQL in concat as well as group_concat Use - Programmer base

MySQL function Locate Use _ Blog without wind or rain tonight -CSDN Blog _mysql Of locate

MySQL Medium substring_index() Function usage methods and skills _ Geek Xiaojun's blog -CSDN Blog _mysql substring_index