Current period time vs previous period time

Hi All,

I have to create a report which compare the sales period time..

for example variable 0calmonth period 0108 - 0208

product | current period 0108 / 0208 | previous period 0107 / 0207 | current - previous

TV | 1000 | 500 | 500

Audio | 100 | 300 | -200

its work great

but when I add 0calmonth in column query

product | current period 0108 - 0208 | previous period 0107 - 0207 | current - previous

0107

0207

0108

0208

0107

0207

0108

0208

0107

0207

0108

0208

TV | 0 | 0 | 800 | 200 | 300 | 200 | 0 | 0 | -300 | -200 | 800 | 200 |

Audio | 0 | 0 | 70 | 30 | 150 | 150 | 0 | 0 | -150 | -150 | 70 | 30 |

This is not what i want.

I want this.

product | current period 0108 - 0208 | previous period 0107 - 0207 | current - previous

0108

0208

0107

0207

0108-0107

0208-0207

TV | 800 | 200 | 300 | 200 | 500 | 0 |

Audio | 70 | 30 | 150 | 150 | -80 | -120 |

How do that?