我们可以在SQL查询中多次使用相同的列名吗

Question

I want to use same column multiple in same query example 6 times in query how to do this to get result ?

            SELECT J.ID , J.U_POST_ID,
            J.TITLE,J.CREATION_DATE,J.STATUS,
            R.FIRST_NAME, R.LAST_NAME,R.CLINICAL_CLINIC_NAME,
            J.REQUIREMENT,J.STATE,J.CITY,J.DESCRIPTION,J.CALL_DUR,J.USER_ID
            FROM  
            df_job_meta M LEFT OUTER JOIN df_job_post J ON
            M.JOB_ID = J.ID
            LEFT OUTER JOIN df_register_users R ON 
            R.ID = J.USER_ID
            WHERE
            J.STATUS='ACTIVE'  AND R.OCCUPATION !='student' AND  M.VALUE IN ('Clinical')  AND M.VALUE IN ('Full Time')   group  BY J.ID DESC

MY TABLE STRUCTURE IS LIKE THIS

  JOB_ID   |META_KEY       | VALUE
1          |  work_hour    | Full Time
1          | profile       | Clinical
1          | pay_scale     | Sharing

Answer 1

I think you have this trouble:

In your query you have add a WHERE clause like this:

M.VALUE IN ('Clinical') AND M.VALUE IN ('Full Time') 

This condition is always FALSE.

You must collapse your condition in:

M.VALUE IN ('Clinical', 'Full Time')

About use of a column multiple: If you want to add in your SELECT field list the same column, you must use alias like this:

SELECT J.id as ID1, J.id as ID2 and so on...

If you want to use the same column in a WHERE condition there's no problem but pay attention to relational operator you use

EDIT

A further explanation about M.VALUE by an example:

Suppose you have a table named Person .

You have a column named sex where you have value F about FEMAIL and M about MALE.

So if you write this query:

SELECT * FROM Person WHERE sex = 'F' and sex = 'M'

you have 0 rows affected

but if you write this query:

SELECT * FROM Person WHERE sex IN ('F', 'M') 

or equivalent

SELECT * FROM Person WHERE sex = 'F' or sex = 'M'

you have all rows in your table

Answer 2

Since you only have one value in your use of the IN function you can replace that with equals and use or with the use of parenthesis - also group by is not ASC or DESC that is for orderby - try:

SELECT J.ID , J.U_POST_ID,
            J.TITLE,J.CREATION_DATE,J.STATUS,
            R.FIRST_NAME, R.LAST_NAME,R.CLINICAL_CLINIC_NAME,
            J.REQUIREMENT,J.STATE,J.CITY,J.DESCRIPTION,J.CALL_DUR,J.USER_ID
            FROM  
            df_job_meta M LEFT OUTER JOIN df_job_post J ON
            M.JOB_ID = J.ID
            LEFT OUTER JOIN df_register_users R ON 
            R.ID = J.USER_ID
            WHERE
            J.STATUS='ACTIVE'  AND R.OCCUPATION !='student' AND  (M.VALUE = 'Clinical'  OR M.VALUE = 'Full Time' group  BY J.ID

Or if you really are looking for both values then just do:

SELECT J.ID , J.U_POST_ID,
            J.TITLE,J.CREATION_DATE,J.STATUS,
            R.FIRST_NAME, R.LAST_NAME,R.CLINICAL_CLINIC_NAME,
            J.REQUIREMENT,J.STATE,J.CITY,J.DESCRIPTION,J.CALL_DUR,J.USER_ID
            FROM  
            df_job_meta M LEFT OUTER JOIN df_job_post J ON
            M.JOB_ID = J.ID
            LEFT OUTER JOIN df_register_users R ON 
            R.ID = J.USER_ID
            WHERE
            J.STATUS='ACTIVE'  AND R.OCCUPATION !='student' AND  M.VALUE IN ('Clinical','Full Time') group  BY J.ID

You can think of the Mysql IN function like PHP's in_array function.