Another newbie query on 2 tables I'm having trouble working out this apparently simple query, and would appreciate any suggestions for a solution using Sqlite.

I have two tables  t1 and t2 :

Eg.,

Table1 :
--------

Key  Data
A    data11
A    data12
A    data13
B    data21
B    data22
B    data23

Table2 :
--------
Key  Data
A    datax
B    datay


And I want to pad out Table2 to be like this  :

Table2 :
--------
Key  Data
A    datax
A    null
A    null
B    datay
B    null
B    null


Ie., So that both tables have the same number of entries for each Key value.

CREATE TABLE T1(key,data);
CREATE TABLE T2(key,data);
INSERT INTO T1 VALUES ("A","data11"),("A","data12"),("A","data13"), ("B","data21"),("B","data22"),("B","data23");
INSERT INTO T2 VALUES ("A","datax"),("B","datay");


I have tried the following :

CREATE TABLE T1(id integer primary key autoincrement,key,data);
CREATE TABLE T2(id integer primary key autoincrement,key,data);
INSERT INTO T2 SELECT NULL,T1.KEY,NULL FROM T1 LEFT JOIN T2 USING (key,id);


This adds too many rows (6 instead of 4). There should be the same number of rows in both tables. 

|1|A|datax
|2|B|datay
|3|A|Null
|4|A|Null
|5|A|Null
|6|B|Null
|7|B|Null
|8|B|Null


The additional records are just padding so that both tables have the same number of rows. This simplifies the task creating the report where there are a variable number of items per key per category. I didn't know how else to do it, and it took quite a while to come up with this solution.

The ID columns are autoincremented as the tables are populated. I have Sqlite 3.13.0. In reality , the 'key' field are project-codes and the 'data' field are object names from 2 repositories. The report is supposed to show what objects are active for a given project and on which repository.
Try:

INSERT INTO T2 
SELECT NULL,T1.KEY,NULL 
FROM T1 
LEFT JOIN T2 USING (key,id)
WHERE T2.KEY IS NULL;


This should filter your join to select only those rows where there is currently no data value in T2.

(This is your original attempt, plus a new WHERE clause).