[mysql] MySQL LEFT JOIN 3 tables

I have 3 tables:

Persons (PersonID, Name, SS)
Fears (FearID, Fear)
Person_Fear (ID, PersonID, FearID)

Now I'd like to list every person with whatever fear is linked to them (can be multiple fears but can also be none). The persons table has to be shown even if a person doesn't have a fear linked to them.

I think I need to do a LEFT JOIN, but my code doesn't seem to work:

SELECT persons.name, 
       persons.ss, 
       fears.fear 
FROM   persons 
       LEFT JOIN fears 
              ON person_fear.personid = person_fear.fearid 

What am I doing wrong here?

This question is related to mysql sql join

The answer is


You are trying to join Person_Fear.PersonID onto Person_Fear.FearID - This doesn't really make sense. You probably want something like:

SELECT Persons.Name, Persons.SS, Fears.Fear FROM Persons
LEFT JOIN Person_Fear
    INNER JOIN Fears
    ON Person_Fear.FearID = Fears.FearID
ON Person_Fear.PersonID = Persons.PersonID

This joins Persons onto Fears via the intermediate table Person_Fear. Because the join between Persons and Person_Fear is a LEFT JOIN, you will get all Persons records.

Alternatively:

SELECT Persons.Name, Persons.SS, Fears.Fear FROM Persons
LEFT JOIN Person_Fear ON Person_Fear.PersonID = Persons.PersonID
LEFT JOIN Fears ON Person_Fear.FearID = Fears.FearID

SELECT p.*, f.Fear
FROM Persons p
LEFT JOIN Person_Fear pf ON pf.PersonID = p.PersonID
LEFT JOIN Fears f ON f.FearID = pf.FearID
ORDER BY p.PersonID

  1. You need to select from the Persons table to ensure you generate a row for every person, whether they have fears or not.
  2. Then you can left join Person_Fear to every person, which will just be NULL if they don't have any entries (as you want).
  3. Finally, you left join Fears on Person_Fear so that you can select the name of the fear.
  4. Optionally, add an order so that each person has all their fears listed together, even if they were added to the Person_Fear table at different times.

Select 
    p.Name,
    p.SS,
    f.fear
From
    Persons p
left join
        Person_Fear pf
    inner join
        Fears f
    on
        pf.fearID = f.fearID
 on
    p.personID = pf.PersonID

Select Persons.Name, Persons.SS, Fears.Fear
From Persons
LEFT JOIN Persons_Fear
ON Persons.PersonID = Person_Fear.PersonID
LEFT JOIN Fears
ON Person_Fear.FearID = Fears.FearID;

try this

    SELECT p.Name, p.SS, f.Fear 
    FROM Persons p 
    LEFT JOIN Person_Fear fp 
    ON p.PersonID = fp.PersonID
    LEFT JOIN Fear f
    ON f.FearID = fp.FearID

Try this definitely work.

SELECT p.PersonID AS person_id,
   p.Name, p.SS, 
   f.FearID AS fear_id,
   f.Fear 
   FROM person_fear AS pf 
      LEFT JOIN persons AS p ON pf.PersonID = p.PersonID 
      LEFT JOIN fears AS f ON pf.PersonID = f.FearID 
   WHERE f.FearID = pf.FearID AND p.PersonID = pf.PersonID

Examples related to mysql

Implement specialization in ER diagram How to post query parameters with Axios? PHP with MySQL 8.0+ error: The server requested authentication method unknown to the client Loading class `com.mysql.jdbc.Driver'. This is deprecated. The new driver class is `com.mysql.cj.jdbc.Driver' phpMyAdmin - Error > Incorrect format parameter? Authentication plugin 'caching_sha2_password' is not supported How to resolve Unable to load authentication plugin 'caching_sha2_password' issue Connection Java-MySql : Public Key Retrieval is not allowed How to grant all privileges to root user in MySQL 8.0 MySQL 8.0 - Client does not support authentication protocol requested by server; consider upgrading MySQL client

Examples related to sql

Passing multiple values for same variable in stored procedure SQL permissions for roles Generic XSLT Search and Replace template Access And/Or exclusions Pyspark: Filter dataframe based on multiple conditions Subtracting 1 day from a timestamp date PYODBC--Data source name not found and no default driver specified select rows in sql with latest date for each ID repeated multiple times ALTER TABLE DROP COLUMN failed because one or more objects access this column Create Local SQL Server database

Examples related to join

Pandas Merging 101 pandas: merge (join) two data frames on multiple columns How to use the COLLATE in a JOIN in SQL Server? How to join multiple collections with $lookup in mongodb How to join on multiple columns in Pyspark? Pandas join issue: columns overlap but no suffix specified MySQL select rows where left join is null How to return rows from left table not found in right table? Why do multiple-table joins produce duplicate rows? pandas three-way joining multiple dataframes on columns