mysql - 仅在一列的 Left Join 上选择 distinct

Question

我正在尝试列出每个客户购买的产品，但如果他们在不同的场合购买了相同的产品，我希望将其排除在外。这是我目前所拥有的:

Select  c.field_id_33  AS email, o.order_id, Group_concat(o.entry_id) AS Products,group_concat(t.title),group_concat(t.url_title) from finn_cartthrob_order_items o  
LEFT JOIN finn_channel_data c
ON c.entry_id=o.order_id
LEFT JOIN finn_channel_titles t
ON o.entry_id=t.entry_id
GROUP BY email

这是生产:

基本上，无论他们购买了多少次，只要他们购买了产品，我就只需要列出一次。我该怎么做？

Answer 1

您可以在 group_concat 函数中使用 DISTINCT，使用 Group_concat意识到这个事实，它有 1024 个字符的默认限制来对它们进行分组，但可以增加

Select  c.field_id_33  AS email, o.order_id, 
Group_concat(DISTINCT o.entry_id) AS Products,
group_concat(DISTINCT t.title),
group_concat(DISTINCT t.url_title)
 from finn_cartthrob_order_items o  
LEFT JOIN finn_channel_data c
ON c.entry_id=o.order_id
LEFT JOIN finn_channel_titles t
ON o.entry_id=t.entry_id
GROUP BY email

From the docs The result is truncated to the maximum length that is given by the group_concat_max_len system variable, which has a default value of 1024. The value can be set higher, although the effective maximum length of the return value is constrained by the value of max_allowed_packet. The syntax to change the value of group_concat_max_len at runtime is as follows, where val is an unsigned integer:

SET [GLOBAL | SESSION] group_concat_max_len = val;