0
85
1
35
2
38
3
19
4
...

SELECT
  @rownum0:=@rownum0+2 rn,
  f.*
FROM ( SELECT * FROM foo ORDER BY id ) f, (SELECT @rownum0:=0) r
UNION
SELECT @rownum1:=@rownum1+2 rn,
  b.*
FROM ( SELECT * FROM bar ORDER BY RAND() ) b, (SELECT @rownum1:=-1) r
ORDER BY rn
LIMIT 100

SELECT * FROM foo ORDER BY id LIMIT 50 UNION SELECT * FROM foo ORDER BY RAND() LIMIT 50

SELECT * FROM foo ORDER BY id LIMIT 50 UNION SELECT * FROM foo ORDER BY RAND() LIMIT 50