Reversed query SQL | 易学教程

问题

The usual way of setting up the query

SELECT * from ____
  WHERE (Column1>0) and ____

is like asking "What are the records that have column1 values > 0 and column2 .....?"

However now I want to do it the opposite way,

"For this record, what are the columns that have values > 0?"

How to write a query that returns column names if the column values meet the condition, given a specified record ID? There are non-integer columns, which should be neglected.

EDIT: My table contains many columns that take only value 0 or 1. I wan to first randomize an ID by some constraints, like

SELECT id from ____
  WHERE Views>5000 and Q1=1 and Q3=1

then do the randomization, before showing and results of which columns = 1.

"Result: ID:_____. Views:____. Q1, Q3, Q6, Q14, Q38, Q45, Q56 exists.(=1)"

Now from the answers it seems that I have to check column by column, rather than using a function that automatically checks all columns, so I think the methodology will have to be the same whether I use SQL or post-processing by PHP. I was just hoping some kind of functions can do this rather than writing loops.

回答1:

You could build a string based on CASE WHEN's or IF's for each column that should be verified if the value is bigger than 0.

For example:

CREATE TABLE test_tbl(
   id INT NOT NULL AUTO_INCREMENT,
   col1 INT,
   col2 INT,
   col3 INT,
   PRIMARY KEY ( id )
   );

insert into test_tbl (col1,col2,col3)
values 
(0,0,0),
(8,0,0),
(8,null,8),
(8,8,8);

select 
*,
TRIM(TRAILING ',' FROM 
  concat(
   if(col1 > 0,'col1,',''),
   case when col2 > 0 then 'col2,' else '' end,
   if(col3 > 0,'col3,','')
  )
) as bigger_than_0
from test_tbl
where (col1>0 or col2>0 or col3>0);

Gives:

id  col1 col2 col3  bigger_than_0
2   8    0    0     col1
3   8    null 8     col1,col3
4   8    8    8     col1,col2,col3

回答2:

There are two possibilities with MySQL that I could think of which are CASE and IF statements, e.g. below

SELECT
id, 
case
 when a.column1 > 0 then 'column1'
 else null
end AS col1,
case
 when a.column2 > 0 then 'column2'
 else null
end AS col2
FROM table_a a;

SELECT 
id, 
IF(column1 > 0, 'column 1', 'null') as col1,
IF(column2 > 0, 'column 2', 'null') as col2
FROM table_a a;

回答3:

This what you want to know :

create table TT(col1 int,col2 int,col3 int);

insert into TT values(1,0,1);
insert into TT values(4,0,5);
insert into TT values(1,0,3);
insert into TT values(1,0,0);

select distinct
   'col1' as 'Column contains values >1'
from 
    TT
where 
    col1 > 0

union

select distinct
    'col2'  as 'Column contains values >1'
from 
    TT
where 
    col2 > 0

union

select distinct
    'col3'  as 'Column contains values >1'
from 
    TT
where  
    col3 > 0;

Result :

+---------------------------+
| Column contains values >1 |
+---------------------------+
| col1                      |
| col3                      |
+---------------------------+

because col1 and col3 have values >0 and you can customize you query however you like

来源：https://stackoverflow.com/questions/38408365/reversed-query-sql

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mysql

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