Question

How do i join 3 or more tables in mysql as follows?

• there is a column for each column of each table (except ID)
• ID field values all go into the same ID field in the new table
• an additional column is added called table the values of which is the source Table name
• an autoincremented newID field is added
• only one table contributes to each row, unrelated fields have null values
• total number of rows is equal to the sum records from all tables

example with just two tables :

TableA:                  TableB          
ID   |   fieldA          ID   |   fieldB 
-----------------       -----------------
  1  |   valueA1           1  |   valueB1
  2  |   valueA2           2  |   valueB2


ResultTable:
newID  | ID  |  table  | fieldA  |  fieldB
---------------------------------------------
   1   |  1  |  TableA | valueA1 |  
   2   |  2  |  TableA | valueA2 |  
   3   |  1  |  TableB |         |  valueB1
   4   |  2  |  TableB |         |  valueB2

I know this probably sounds a bit weird!. I am going to try and use this to batch insert nodes for records from various tables into neojs graph database with this batch-insert script. which could be hilarious considering I hardly know what I am doing in either database 😉 .

Answer 1

Try this one,

SELECT  @rownum := @rownum + 1 AS NewID,
        a.*
FROM
    (
        SELECT  ID, fieldA, '' AS fieldB
        FROM    tableA
        UNION ALL
        SELECT  ID, '' AS fieldA, fieldB
        FROM    tableB
    ) a, (SELECT @rownum:=0) r

SQLFiddle Demo

• Create New Table

here’s the proposed schema

CREATE TABLE Newtable
(
    NewID INT AUTO_INCREMENT,
    ID INT NOT NULL,
    FieldA VARCHAR(30),
    FieldB Varchar(30),
    CONSTRAINT tb_pk PRIMARY KEY (NewID)
)

• then Insert your values,

here’s the query using INSERT INTO...SELECT statement

INSERT INTO NewTable (ID, fieldA, fieldB)
SELECT  ID, fieldA, NULL AS fieldB
FROM    tableA
UNION ALL
SELECT  ID, NULL AS fieldA, fieldB
FROM    tableB