MySQL query from multiple databases and select random - mysql

Question

I would like to select random entries from a single table based on information coming from two other tables (one saved in a different database). The tables are as follows:
1- In databaseA called "islands" contains:
+-------------+------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| chrom | int(11) | NO | | NULL | |
| start | int(11) | NO | | NULL | |
| end | int(11) | NO | | NULL |
the indexes are:
+---------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| islands | 0 | PRIMARY | 1 | id | A | 15991 | NULL | NULL | | BTREE | | |
| islands | 1 | locations | 1 | line_string | A | NULL | 32 | NULL | | SPATIAL | | |
To select from this database I normally use:
SELECT * FROM islands FORCE INDEX (locations)
WHERE MBRIntersects(GeomFromText('Linestring(1 120, 1 120)'), line_string)
2 - In databaseB call "Context" contains:
+---------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------+---------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| chrom | int(11) | NO | | NULL | |
| site | int(11) | NO | | NULL | |
| context | char(3) | NO | | NULL | |
There are only 4 possible contexts in this table (which are indexed)
3 - In databaseB called "Entries" Contains:
+-------------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+---------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| chrom | int(11) | NO | MUL | NULL | |
| site | int(11) | NO | MUL | NULL | |
| methylation | float | NO | | NULL | |
I would like to select a random entry from the entries table that is located in the context table with a context entry of "CpG", can be located in the "islands" table or not (I have two different searches).
Update:
Based on comments below I am using
SELECT * FROM
(SELECT t.chrom, t.site, t.methylation, c.context
FROM f1 as t INNER JOIN context as c on c.chrom = t.chrom AND c.site = t.site
WHERE c.context = 'CpG'
) AS s LIMIT 2;
To get:
+-------+-----------+-------------+---------+
| chrom | site | methylation | context |
+-------+-----------+-------------+---------+
| 1 | 10003735 | 69 | CpG |
| 1 | 100063074 | 98.79 | CpG |
+-------+-----------+-------------+---------+
I would like to now join these results to the islands table to get the sites from the first part that are found within island regions (or not). I am using:
SELECT * FROM
(SELECT t.chrom, t.site, t.methylation, c.context
FROM f1 as t INNER JOIN context as c on c.chrom = t.chrom AND c.site = t.site
WHERE c.context = 'CpG'
) AS s
CROSS JOIN islands as i
WHERE MBRINTERSECTS(GeomFromText('Linestring(s.chrom s.site, s.chrom s.site)'), i.Line_string)
LIMIT 2;
However, this gives me an empty set (it shouldn't).