未定义的偏移量:在Yii Controller中使用子查询后为0

在Controller中使用子查询后出现问题？这是我的子查询：

"select "
. " dat_mahasiswa.nim_mhs, dat_semester.ket_smt, case left(dat_mahasiswa.kd_kls,1) 
when '0' then mid(dat_mahasiswa.kd_kls,2,1) else mid(dat_mahasiswa.kd_kls,1,2) 
end semt, nama_mhs, no_uts, tab_shift.ket_shift, dat_mahasiswa.kd_kls, 
id_krs, dat_krs.kd_kls krs_kls, id_jdw, tgl_uts tgl_ujian, wkt_uts wkt_ujian,
concat(case DAYNAME(dat_jadwal_kuliah.tgl_uts) when 'Sunday' then 'MINGGU' 
else case DAYNAME(dat_jadwal_kuliah.tgl_uts) when 'Monday' then 'SENIN' 
else case DAYNAME(dat_jadwal_kuliah.tgl_uts) when 'Tuesday' then 'SELASA' 
else case DAYNAME(dat_jadwal_kuliah.tgl_uts) when 'Wednesday' then 'RABU' 
else case DAYNAME(dat_jadwal_kuliah.tgl_uts) when 'Thursday' then 'KAMIS' 
else case DAYNAME(dat_jadwal_kuliah.tgl_uts) when 'Friday' then 'JUM''AT' 
else 'SABTU' end end end end end end,', ',
DATE_FORMAT(dat_jadwal_kuliah.tgl_uts, '%d %M %Y')) 'HARI/TANGGAL', 
dat_jadwal_kuliah.wkt_uts 'WAKTU', dat_jadwal_kuliah.kd_ruang_uts 'RUANG', 
dat_jadwal_kuliah.kd_kls 'KELAS', tab_mata_kuliah.nama_mt_klh 
'NAMA MATA KULIAH', '' as PARAF, no_uts From dat_mahasiswa 
INNER JOIN dat_krs ON dat_mahasiswa.nim_mhs = dat_krs.nim_mhs 
INNER JOIN dat_jadwal_kuliah ON (dat_krs.kd_kls = dat_jadwal_kuliah.kd_kls 
and dat_krs.kd_smt=dat_jadwal_kuliah.kd_smt) 
INNER JOIN tab_shift ON dat_mahasiswa.kd_shift = tab_shift.kd_shift 
INNER JOIN dat_semester ON dat_krs.kd_smt=dat_semester.kd_smt 
INNER JOIN tab_mata_kuliah ON dat_krs.kd_mt_klh = tab_mata_kuliah.kd_mt_klh 
INNER JOIN tab_program_studi ON dat_krs.kd_pst = tab_program_studi.kd_pst 
INNER JOIN dat_dosen dosen1 ON tab_program_studi.ketua_pst = dosen1.ni_dos 
INNER JOIN dat_panitia_ujian ON dat_panitia_ujian.kd_smt = dat_jadwal_kuliah.kd_smt
INNER JOIN dat_no_ujian ON (dat_krs.nim_mhs = dat_no_ujian.nim_mhs 
and dat_krs.kd_smt = dat_no_ujian.kd_smt) 
INNER JOIN (select nim_mhs from keu_tagihan where id_jns_tghn=3 
             and jml_bayar>=(jml_tghn/2) 
             and kd_smt='$_smt' 
             and nim_mhs='$_nim'
) tagihansks on dat_mahasiswa.nim_mhs=tagihansks.nim_mhs 
INNER JOIN (select nim_mhs from keu_tagihan where id_jns_tghn=5 
             and jml_bayar>=jml_tghn 
             and kd_smt='$_smt' 
             and nim_mhs='$_nim'
) tagihanuts on dat_mahasiswa.nim_mhs=tagihanuts.nim_mhs 
where dat_mahasiswa.kd_shift <> 'E' and dat_mahasiswa.nim_mhs='$_nim' 
and tab_mata_kuliah.mt_ujian='Y' and dat_krs.kd_smt='$_smt' 
AND dat_jadwal_kuliah.kd_mt_klh = dat_krs.kd_mt_klh 
and dat_jadwal_kuliah.kd_pst=tab_mata_kuliah.kd_pst ;";

我有这样的错误

错误500

未定义偏移：0

谢谢你。