Combine two statements with LIMITS using UNION - sql

Question

Is there a way to combine these two statements into one without having duplicate entries?
SELECT * FROM Seq where JULIANDAY('2012-05-25 19:02:00')<=JULIANDAY(TimeP)
order by TimeP limit 50
SELECT * FROM Seq where JULIANDAY('2012-05-29 06:20:50')<=JULIANDAY(TimeI)
order by TimeI limit 50
My first, obvious attempt is not supported by SQLITE (Syntax error: Limit clause should come after UNION not before):
SELECT * FROM Seq where JULIANDAY('2012-05-25 19:02:00')<=JULIANDAY(TimeP)
order by TimeP limit 50
UNION
SELECT * FROM Seq where JULIANDAY('2012-05-29 06:20:50')<=JULIANDAY(TimeI)
order by TimeI limit 50

Answer 1

Use subqueries and perform the limit within them.
SELECT *
FROM ( SELECT *
FROM Seq
WHERE JULIANDAY('2012-05-25 19:02:00') <= JULIANDAY(TimeP)
ORDER BY TimeP
LIMIT 50
)
UNION
SELECT *
FROM ( SELECT *
FROM Seq
WHERE JULIANDAY('2012-05-29 06:20:50') <= JULIANDAY(TimeI)
ORDER BY TimeI
LIMIT 50
)

Answer 2

Queries are processed in stages:
FROM clause and all the joins;
WHERE clause and all the predicates. So if you whant to see NULL values in the result set, you should never filter OUTER-joined table columns in the WHERE section, as this will turn your query into INNER join;
GROUP BY and HAVING clause;
Query combinations: UNION, INTERSECT, EXCEPT or MINUS
ORDER BY
LIMIT
Therefore, as others pointed out, it is syntatically wrong to use ORDER BY and LIMIT before UNION clause. You should use subqueries:
SELECT *
FROM (SELECT * FROM Seq
WHERE JULIANDAY('2012-05-25 19:02:00') <= JULIANDAY(TimeP)
ORDER BY TimeP LIMIT 50) AS tab1
UNION
SELECT *
FROM (SELECT * FROM Seq
WHERE JULIANDAY('2012-05-29 06:20:50') <= JULIANDAY(TimeI)
ORDER BY TimeI LIMIT 50) AS tab2;

Answer 3

SELECT * from
(SELECT *
FROM Seq
where JULIANDAY('2012-05-25 19:02:00')<=JULIANDAY(TimeP)
order by TimeP limit 50)
UNION
SELECT * from
(SELECT *
FROM Seq
where JULIANDAY('2012-05-29 06:20:50')<=JULIANDAY(TimeI)
order by TimeI limit 50)

Answer 4

I have a table buysell_product. I want to select top 5 based on views_1 column and another top 5 using views_2 column, then merge the two. Columns are the same in both queries. Expecting this to work, it does not:
SELECT id, name, views_1, views_2
FROM buysell_product
ORDER BY views_1 DESC
LIMIT 5
UNION
SELECT id, name, views, views_2
FROM buysell_product as b
ORDER BY views_2 DESC
LIMIT 5
Error:
Execution finished with errors.
Result: ORDER BY clause should come after UNION not before
At line 1:
...
They work separately but I need to merge them:
SELECT * FROM (
SELECT id, views_1, views_2, name
FROM buysell_product
ORDER BY views_1 DESC
LIMIT 5
)
UNION
SELECT * FROM (
SELECT id, views_1, views_2, name
FROM buysell_product as b
ORDER BY views_2 DESC
LIMIT 5
)
Result:
id
views_1
views_2
name
2
41
16
Excellent 2013 ford ecosport
3
72
10
Excellent Hyundai creta
5
39
39
iPhone 11 128gb
7
12
84
Excellent Hyundai creta sx
9
37
84
Volkswagen Polo 1.2 GT AMT 2017
44
34
81
Usupso Massage Anti Skid Slippers
45
15
75
Garlic Powder - 100Gm
57
35
11
Iphone 13 and 14
67
15
73
Universal Touch Screen Capacitive Stylus