クエリ全体は次のようになります。ここで、*を選択する列名に置き換えます!
INSERT INTO table1(first,second,third)
-- replace * with columns name first,second,third
select * from (
-- START YOU'RE select query
(SELECT table2.timemodified FROM `xtable` AS table2,`ytable` AS table3
WHERE table3.id = table2.contextid),
(SELECT table4.id FROM `ztable` AS table4,`ytable` AS table3 WHERE table4.id = table3.instanceid),
(SELECT murs.id FROM `table5` AS murs,
`xtable` AS table2,
`wtable` AS table6,
`ytable` AS table3,
`vtable` AS table7
WHERE murs.id = table2.userid AND table6.id = table2.roleid AND table3.id = table2.contextid AND table7.instance = table3.instanceid AND table6.id =3)
-- END YOU'RE select query
)
selectステートメントをサブクエリに移動したので、サブクエリの合計結果を有利に使用できます。