mysql select column default value if is null

mysql select column default value if is null

SELECT
w.city AS city,
w.city_en AS city_en,
w.date_y AS date_y,
w.week AS week,
w.cityid AS cityid,
w.temp1 AS temp1,
w.temp2 AS temp2,
w.temp3 AS temp3,
w.temp4 AS temp4,
w.temp5 AS temp5,
w.temp6 AS temp6,
w.weather1 AS weather1,
w.weather2 AS weather2,
w.weather3 AS weather3,
w.weather4 AS weather4,
w.weather5 AS weather5,
w.weather6 AS weather6,
w.wind1 AS wind1,
w.wind2 AS wind2,
w.wind3 AS wind3,
w.wind4 AS wind4,
w.wind5 AS wind5,
w.wind6 AS wind6,
w.fl1 AS fl1,
w.fl2 AS fl2,
w.fl3 AS fl3,
w.fl4 AS fl4,
w.fl5 AS fl5,
w.fl6 AS fl6,
w.ind AS ind,
w.index_xs AS index_xs,
w.index_uv AS index_uv,
w.index_xc AS index_xc,
w.index_tr AS index_tr,
w.index_co AS index_co,
w.index_cl AS index_cl,
w.index_ls AS index_ls,
w.index_ag AS index_ag,
w.create_time AS create_time,
sk.temp AS temp,
sk.wd AS wd,
sk.ws AS ws,
sk.sd AS sd,
sk.wse AS wse,
sk.time AS time,
sk.create_time AS create_time_sk,
COALESCE(air.aqi,0) AS aqi,
COALESCE(air.level,'') AS level,
COALESCE(air.position_name,'') AS position_name
FROM (SELECT * FROM ep_weather_report_current WHERE city='汝州') w
LEFT JOIN ep_weather_sk_temperature_current sk ON w.cityid = sk.cityid
LEFT JOIN (SELECT * FROM ep_airpollution_realtime_current WHERE city = position_name AND city = '汝州') air
ON w.cityid = air.cityid

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COALESCE(air.aqi,0) AS aqi,
COALESCE(air.level,'') AS level,
COALESCE(air.position_name,'') AS position_name