about query of installed package

[ykcheung.bbs@VOIDbbs.e-fever.org (把我的夢搖醒了)]

i would like to ask how can how query the installed 
package names.

i redhat, i can use "rpm -qa | grep xxxx" or "rpm -qa"
but how to perform similar operation in debian?

i find it time consuming to search in dselect...

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