当我加入一个空表并将其添加到 kills_total 字段时,kills_total 值显示 NULL。
SELECT
alias.name alias, team.name team, sid.steam_id steam_id,
(SUM(ws1.kills) + SUM(ws2.kills) + SUM(spnr.kills)) kills_total
FROM pickup
JOIN player ON player.pickup_id = pickup.id
JOIN team ON player.team_id = team.id
JOIN sid ON player.sid_id = sid.id
JOIN alias ON player.alias_id = alias.id
LEFT JOIN weapon_stats_1 ws1 ON ws1.pickup_id = pickup.id AND ws1.player_id = player.id
LEFT JOIN weapon_stats_2 ws2 ON ws2.pickup_id = pickup.id AND ws2.player_id = player.id
LEFT JOIN spanner_stats spnr ON spnr.pickup_id = pickup.id AND spnr.player_id = player.id
WHERE pickup.logfile_name = 'srv-20130725-2151-log' GROUP BY player.id ORDER BY kills_total DESC
这是结果:
SELECT
alias.name alias, team.name team, sid.steam_id steam_id,
(SUM(ws1.kills) + SUM(ws2.kills)) kills_total
FROM pickup
JOIN player ON player.pickup_id = pickup.id
JOIN team ON player.team_id = team.id
JOIN sid ON player.sid_id = sid.id
JOIN alias ON player.alias_id = alias.id
LEFT JOIN weapon_stats_1 ws1 ON ws1.pickup_id = pickup.id AND ws1.player_id = player.id
LEFT JOIN weapon_stats_2 ws2 ON ws2.pickup_id = pickup.id AND ws2.player_id = player.id
WHERE pickup.logfile_name = 'srv-20130725-2151-log' GROUP BY player.id ORDER BY kills_total DESC
这是我从查询中删除空表时的结果:
我如何加入 spanner_stats 表,如果它是空的,什么都不做(即 SUM(0)),所以 ws1 和 ws2 即使正在连接的第三个表没有匹配 "thirdTable.pickup_id = pickup.id AND thirdTable.player_id = player .id"?
最佳答案
试试这个:
SELECT
alias.name alias, team.name team, sid.steam_id steam_id,
(SUM(IFNULL(ws1.kills,0)) + SUM(IFNULL(ws2.kills,0)) + SUM(IFNULL(spnr.kills,0))) kills_total
FROM pickup
JOIN player ON player.pickup_id = pickup.id
JOIN team ON player.team_id = team.id
JOIN sid ON player.sid_id = sid.id
JOIN alias ON player.alias_id = alias.id
LEFT JOIN weapon_stats_1 ws1 ON ws1.pickup_id = pickup.id AND ws1.player_id = player.id
LEFT JOIN weapon_stats_2 ws2 ON ws2.pickup_id = pickup.id AND ws2.player_id = player.id
LEFT JOIN spanner_stats spnr ON spnr.pickup_id = pickup.id AND spnr.player_id = player.id
WHERE pickup.logfile_name = 'srv-20130725-2151-log' GROUP BY player.id ORDER BY kills_total DESC
本质上,如果左连接没有记录,您是在告诉 MySql 使用 0 而不是 NULL
。看看 IFNULL() http://www.w3schools.com/sql/sql_isnull.asp .
我相信,如果任何组件为 NULL,SQL SUM() 函数将返回 NULL。
关于mysql - 空表上的 LEFT JOIN NULL 结果,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/17872743/