条件表达式

CASE WHEN condition1 THEN result1
            [WHEN condition2 THEN result2]
            [ELSE result3]
END

* |
SELECT
  CASE
    WHEN http_user_agent like '%Chrome%' then 'Chrome'
    WHEN http_user_agent like '%Safari%' then 'Safari'
    ELSE 'unknown'
  END AS http_user_agent,
  count(*) AS pv
GROUP BY
  http_user_agent

* |
SELECT
  CASE
    WHEN request_time < 0.001 then 't0.001'
    WHEN request_time < 0.01 then 't0.01'
    WHEN request_time < 0.1 then 't0.1'
    WHEN request_time < 1 then 't1'
    ELSE 'overtime'
  END AS request_time,
  count(*) AS pv
GROUP BY
  request_time

* |
SELECT
  sum(IF(status = 200, 1, 0)) * 1.0 / count(*) AS status_200_percentag

NULLIF(expression1, expression2)

* | SELECT NULLIF(server_addr,http_host)

TRY(expression)

* | 
SELECT
  TRY(regexp_extract(uri, '.*\\/(index.*)', 1)) 
    AS file, count(*) 
    AS count 
GROUP BY 
  file

COALESCE(expression1, expression2...)

* | select COALESCE(null, 'test')