plugins - Second dropdown changes after select first dropdown in joomla xml -

i making these dropdown list in xml plugin:- category multi-select dropdown, subcategory multi-select dropdown .i want when select first category dropdown second dropdown list come under selected category on first dropdown. example:-

<field name="category- list" type="sql" default="" label="category" sql_select="e.*" sql_from="#__category e" sql_group="category" sql_order="e.cat_id asc" key_field="cat_id" value_field="category" multiple="true"  /> <field name="subcategory - list depend on category list" default="" type="sql" label="subcategory" sql_select="e.*" sql_from="#__subcategory e" sql_group="subcategory" sql_order="e.subcat_id asc" sql_filter="cat_id" key_field="subcat_id" value_field="subcategory" multiple="true"  /> 

create table category ( cat_id int(2) not null auto_increment, category varchar(25) not null default '', primary key (cat_id) ) create table if not exists subcategory ( subcat_id int(3) not null auto_increment, cat_id int(2) not null default '0', subcategory varchar(25) not null default '', unique key subcat_id (subcat_id) )