SQL中case用法

https://www.tablesgenerator.com/markdown_tables#

case 语法

case具有两种格式。

1.简单case函数:

WHEN when_expression 
THEN result_expression [ ...n ]   
[ ELSE else_result_expression ]   
END   

2.case搜索函数:

CASE  
WHEN Boolean_expression 
THEN result_expression [ ...n ]    
[ ELSE else_result_expression ]   
END  

3.实例：

以下是我在seagate时遇到的情况：

已知表status的结构如下：

Column	Modifiers
script_msg	character varying(64)
family	character varying(16)
cell_sn	character varying(24)
send_parm	smallint
disabled	integer
active_config	character varying(64)
results_format_mask	smallint
load_permissions	character varying(32)
cell_type	character varying(8)
batch_name	character varying(32)
part_num	character varying(32)
row_num	smallint
temperature	double precision
restart_flags	text
user_state	character varying(16)
jiffy_count	integer
cm_index	smallint
hda_sn	character varying(16)
status_change_ts	double precision
do_not_start	smallint
wattage	double precision
physical_cell	integer
carrier_sn	character varying(16)
drive_power	integer
discovery_stat	character varying(16)
maptime	integer
fis_rim_type	character varying(8)
slot_state	character varying(16)
end_time	double precision
drive_pairing_cur	character varying(32)
drive_pairing_req	character varying(32)
local_config	character varying(64)
cool_demand	integer
operation	character varying(16)
cell_status	character varying(16)
start_time	double precision
cell_online	character varying(8)
slot_batch_status	character varying(32)
wait_state_time	double precision
drive_fan	smallint
grid_name	character varying(64)
automation_reason	character varying(256)
script_results_dir	character varying(64)
replug_req_time	double precision
script_error	character varying(10)
sbr	character varying(32)
static_rim_type	character varying(32)
run_type	character varying(64)
script_info	character varying(64)
cm_ip_addr	character varying(16)
drive_grade	character varying(16)
pack_id	character varying(16)
adaptor_sn	character varying(16)
matco_state	character varying(32)
cm_status	character varying(8)
port_index	smallint
tray_index	smallint
heat_demand	integer
unload_time	double precision
cell_fan	smallint
column_num	smallint
rim_type	character varying(32)

此时我想要知道在表格status中testing, pass, fail,restart四种状态的数量。
如下代码所示：

select 
sum(case 
		when cell_status='TESTING' 
			then 1 
			else 0 
		end) 
as TESTING,
sum(case 
	when cell_status='PASS' 
		then 1 
		else 0 
	end) 
as PASS,
sum(case 
		when cell_status='FAIL' 
			then 1 
			else 0 
		end) 
as FAIL,
sum(case 
		when cell_status='RESTART' 
			then 1 
			else 0 
		end) as RESTART 
from status;