MySQL UNION not working - mysql

Question

I'm attempting a mysql union but nothing is returned? Below is an example. In total there is 16 select queries
$url_array = array(
"euro-gbp","euro-aud","euro-usd",
"euro-jpy","gbp-jpy","euro-cad",
"usd-cad","usd-jpy","cad-jpy",
"gbp-usd","aud-usd","gbp-cad",
"aud-cad","aud-jpy","aud-nzd",
"euro-nzd","gbp-aud","gbp-nzd",
"nzd-usd","nzd-cad","nzd-jpy");
foreach($url_array as $urls) {
$sql[]= "SELECT *
FROM `data_analysis_child`
WHERE type='".$urls."'
ORDER BY id DESC
LIMIT 2";
}
$sql = implode(" UNION ",$sql);
The sql result
SELECT * FROM `data_analysis_child` WHERE type='euro-gbp' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='euro-aud' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='euro-usd' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='euro-jpy' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='gbp-jpy' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='euro-cad' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='usd-cad' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='usd-jpy' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='cad-jpy' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='gbp-usd' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='aud-usd' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='gbp-cad' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='aud-cad' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='aud-jpy' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='aud-nzd' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='euro-nzd' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='gbp-aud' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='gbp-nzd' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='nzd-usd' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='nzd-cad' ORDER BY id DESC LIMIT 2 UNION SELECT * FROM `data_analysis_child` WHERE type='nzd-jpy' ORDER BY id DESC LIMIT 2
How do i solve? Is there a better way to restructure this query?

Answer 1

if you need order by and limit for each select you should use () around each select
foreach($url_array as $urls) {
$sql[]= " ( SELECT * FROM `data_analysis_child` WHERE type='".$urls."' ORDER BY id DESC LIMIT 2 )";
break;
}

Answer 2

Just a little addition to do that. If you are getting ZERO results then use UNION ALL instead of only UNION. Actually duplicate records are avoided in UNION but with UNION ALL you will get everything that a query is returning. give it a try !!!

Answer 3

The documentation of UNION clearly explains:
To apply ORDER BY or LIMIT to an individual SELECT, place the clause inside the parentheses that enclose the SELECT:
(SELECT a FROM t1 WHERE a=10 AND B=1 ORDER BY a LIMIT 10)
UNION
(SELECT a FROM t2 WHERE a=11 AND B=2 ORDER BY a LIMIT 10);
It also says:
Note
Previous versions of MySQL may permit such statements without parentheses. In MySQL 5.7, the requirement for parentheses is enforced.
If you are using MySQL 5.7 or newer, the code doesn't return anything because the query doesn't run. You should check the type of the returned value (the query functions return FALSE when the query is invalid) and the error message.

Answer 4

You could do this:
SELECT *
FROM (
SELECT #rank := if (#last_type=type, #rank+1, 1) as type_rank, *
FROM data_analysis_child
WHERE type IN ('euro-gbp', 'euro-aud', 'euro-usd'....)
ORDER BY id DESC
)
WHERE type_rank<=2;
But the performance will suck.
Depending on how the indexes are configured and assuming that your id column is unique this might be better:
SELECT 'first', dac1.*
FROM data_analysis_child dac1
WHERE dac1.id IN (
SELECT MAX(dac2.id)
FROM data_analysis_child dac2
WHERE dac2.type IN ('euro-gbp', 'euro-aud', 'euro-usd'....)
GROUP BY dac2.type
)
UNION
SELECT 'second', dac3.*
FROM data_analysis_child dac3
WHERE dac3.id IN (
SELECT MAX(dac4.id)
FROM data_analysis_child dac4
WHERE dac4.type IN ('euro-gbp', 'euro-aud', 'euro-usd'....)
AND dac4.id NOT IN (
SELECT MAX(dac5.id)
FROM data_analysis_child dac5
WHERE dac5.type IN ('euro-gbp', 'euro-aud', 'euro-usd'....)
GROUP BY dac5.type
)
GROUP BY dac4.type
)
But it does not scale well to getting the top N values for each type.
A better solution would be to to flag the state of the first or second rank as an attribute on the record and filter directly.