分步获取：

$sql1="select count(*) countall from visit_looyu_chat where 1=1";

$sql2="select count(*) countuser from visit_looyu_chat where 1=1 and chat_startmode=2";

$sql3="select count(*) countavail from visit_looyu_chat where 1=1 and chat_usermsgcount>0 and chat_viewermsgcount>0";

使用case when：

$sql="select count(*) countall
,sum(case when chat_startmode=2 then 1 else 0 end) as countuser
,sum(case when chat_usermsgcount>0 and chat_viewermsgcount>0 then 1 else 0 end) as countavail
from visit_looyu_chat";