空表上的LEFT JOIN结果为NULL

Question

当我加入一个空的表并将其添加到 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，如果它为空，则什么都不做（即SUM（0）），所以 ws1 和 ws2 值为SUM（）med up并显示在 kills_total 中，即使正在连接的第三个表没有单行匹配“thirdTable.pickup_id = pickup.id AND thirdTable.player_id = player .ID“

Answer 1

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

如果左连接没有记录，基本上你告诉MySql使用0而不是NULL。看看IFNULL（）http://www.w3schools.com/sql/sql_isnull.asp。

如果任何组件为NULL，则SQL SUM（）函数返回NULL。我相信。

Answer 2

也许您可以尝试使用IFNULL()

  

<强> IFNULL（表达式1，表达式2）

     

如果expr1不为NULL，则IFNULL（）返回expr1;否则它会返回   表达式2。 IFNULL（）返回一个数字或字符串值，具体取决于   使用它的上下文。

看一下下面的例子来说明差异。

SQL Fiddle DEMO