How to select first x records from second table - sql

Question

I would like to get all records from first table and only x records from second table.
How many records from second table I have info in first table :
My tables are
table1 :
WITH table1(a,b) AS
(
SELECT 'aa',3 FROM dual UNION ALL
SELECT 'bb',2 FROM dual UNION ALL
SELECT 'cc',4 FROM dual
)
SELECT *
FROM table1;
a | b (number of records from table2 (x))
------
aa | 3
bb | 2
cc | 4
table2 :
WITH table2(a,b) AS
(
SELECT 'aa','1xx' FROM dual UNION ALL
SELECT 'aa','2yy' FROM dual UNION ALL
SELECT 'aa','3ww' FROM dual UNION ALL
SELECT 'aa','4zz' FROM dual UNION ALL
SELECT 'aa','5qq' FROM dual UNION ALL
SELECT 'bb','1aa' FROM dual UNION ALL
SELECT 'bb','2bb' FROM dual UNION ALL
SELECT 'bb','3cc' FROM dual UNION ALL
SELECT 'cc','1oo' FROM dual UNION ALL
SELECT 'cc','2uu' FROM dual UNION ALL
SELECT 'cc','3tt' FROM dual UNION ALL
SELECT 'cc','4zz' FROM dual UNION ALL
SELECT 'cc','5rr' FROM dual
)
SELECT *
FROM table2;
a | b
--------
aa | 1xx
aa | 2yy
aa | 3ww
aa | 4zz
aa | 5qq
bb | 1aa
bb | 2bb
bb | 3cc
bb | 4dd
bb | 5ee
cc | 1oo
cc | 2uu
cc | 3tt
cc | 4zz
cc | 5rr
Expected Result:
a | b
--------
aa | 1xx
aa | 2yy
aa | 3ww
bb | 1aa
bb | 2bb
cc | 1oo
cc | 2uu
cc | 3tt
cc | 4zz

Answer 1

You can use ROW_NUMBER() analytic function with LEFT/RIGHT OUTER JOIN among the tables :
WITH t2 AS
(
SELECT t2.a,t2.b, ROW_NUMBER() OVER (PARTITION BY t2.a ORDER BY t2.b) AS rn
FROM table2 t2
)
SELECT t2.a, t2.b
FROM t2
LEFT JOIN table1 t1
ON t1.a = t2.a
WHERE rn <= t1.b
Demo

Answer 2

You need to write something like:
SELECT a,
b
FROM Table2 T,
( SELECT LEVEL L FROM DUAL
CONNECT BY LEVEL <= (SELECT MAX(b) FROM Table1)
) A
WHERE T.b>= A.L
ORDER BY T.a;

Answer 3

Ideally you should have a ordering column in table2. When you say first X rows it does not make any sense unless you have something like an id or date field to order the records.
Anyway, assuming the number part of the column b in table 2 for ordering and assuming the number would be followed by 2 characters only such as xx,yy etc you can use the logic below
Select Tb1.a, Tb1.b
from
(Select t.*, row_number() over (partition by a order by substr(b,1,length(b)-2)) as seq
from Table2 t) Tb1
join Table1 Tb2
on Tb1.a = Tb2.a
Where Tb1.seq <= Tb2.b;
Demo - https://dbfiddle.uk/?rdbms=oracle_11.2&fiddle=3030b2372bcbb007606bbb6481af9884

Answer 4

Again it's just a job for lateral:
WITH prep AS
(
SELECT *
FROM tab1,
LATERAL
(
SELECT LEVEL AS lvl
FROM dual
CONNECT BY LEVEL <= b
)
)
SELECT p.a, t2.b
FROM prep p
JOIN tab2 t2
ON p.lvl = regexp_substr(t2.b,'^\d+')
AND p.a = t2.a
ORDER BY p.a, p.lvl