1

假设我创建了 3 个表，如下所示...

create table `users` (`username` varchar(20), `other_non_relevant_field` varchar(20) );
insert into `users` values
("Bob","blah blah blah"),
("Steve","blah blah blah"),
("Sue","blah blah blah"),
("Adam","blah blah blah");


create table `table_1` (`username` varchar(20), `field_abc` varchar(20), `field_def` varchar(20), `other_non_relevant_field` varchar(20) );
insert into `table_1` values
("Steve","blue","brown","blah blah blah"),
("Sue","yellow","brown","blah blah blah"),
("Sue","pink","brown","blah blah blah"),
("Adam","green","brown","blah blah blah");


create table `table_2` (`username` varchar(20), `field_ghi` varchar(20), `field_jkl` varchar(20), `other_non_relevant_field` varchar(20) );
insert into `table_2` values
("Bob","spoon","fork","blah blah blah"),
("Bob","knife","spork","blah blah blah"),
("Steve","spoon","knife","blah blah blah"),
("Sue","spoon","fork","blah blah blah"),
("Sue","knife","spork","blah blah blah"),
("Adam","fork","knife","blah blah blah");

然后我运行这个查询......

SELECT users.username,
    table_1.field_abc, table_1.field_def,
    table_2.field_ghi, table_2.field_jkl
FROM users
JOIN table_1 ON table_1.username = users.username
JOIN table_2 ON table_2.username = users.username
WHERE
    table_1.field_abc REGEXP "(spork|pink)" OR
    table_1.field_def REGEXP "(spork|pink)" OR
    table_2.field_ghi REGEXP "(spork|pink)" OR
    table_2.field_jkl REGEXP "(spork|pink)"
ORDER BY
(
    ( CASE WHEN table_1.field_abc LIKE "%spork%" THEN 1 ELSE 0 END ) +
    ( CASE WHEN table_1.field_abc LIKE "%pink%" THEN 1 ELSE 0 END ) +
    ( CASE WHEN table_1.field_def LIKE "%spork%" THEN 1 ELSE 0 END ) +
    ( CASE WHEN table_1.field_def LIKE "%pink%" THEN 1 ELSE 0 END ) +
    ( CASE WHEN table_2.field_ghi LIKE "%spork%" THEN 1 ELSE 0 END ) +
    ( CASE WHEN table_2.field_ghi LIKE "%pink%" THEN 1 ELSE 0 END ) +
    ( CASE WHEN table_2.field_jkl LIKE "%spork%" THEN 1 ELSE 0 END ) +
    ( CASE WHEN table_2.field_jkl LIKE "%pink%" THEN 1 ELSE 0 END )
)DESC;

为什么它不返回 Bob 包含“spork”的条目？你可以在http://sqlfiddle.com/#!2/cbbda/5看到这个查询

不用说，这不是我的实际代码，但它足够相似并且具有相同的问题，我将能够找出我的问题所在。

4

2 回答 2

3

由于默认值JOIN是INNER，查询不返回表返回的结果NULL。使用LEFT JOIN返回“Bob”行，NULL包括值。

http://sqlfiddle.com/#!2/cbbda/7/0

这是各种 JOIN 类型的可视化表示：http: //www.codeproject.com/Articles/33052/Visual-Representation-of-SQL-Joins

SELECT users.username,
  table_1.field_abc, table_1.field_def,
  table_2.field_ghi, table_2.field_jkl
FROM users
LEFT JOIN table_1 ON table_1.username = users.username
LEFT JOIN table_2 ON table_2.username = users.username
WHERE
  table_1.field_abc REGEXP "(spork|pink)" OR
  table_1.field_def REGEXP "(spork|pink)" OR
  table_2.field_ghi REGEXP "(spork|pink)" OR
  table_2.field_jkl REGEXP "(spork|pink)"
ORDER BY
(
  ( CASE WHEN table_1.field_abc LIKE "%spork%" THEN 1 ELSE 0 END ) +
  ( CASE WHEN table_1.field_abc LIKE "%pink%" THEN 1 ELSE 0 END ) +
  ( CASE WHEN table_1.field_def LIKE "%spork%" THEN 1 ELSE 0 END ) +
  ( CASE WHEN table_1.field_def LIKE "%pink%" THEN 1 ELSE 0 END ) +
  ( CASE WHEN table_2.field_ghi LIKE "%spork%" THEN 1 ELSE 0 END ) +
  ( CASE WHEN table_2.field_ghi LIKE "%pink%" THEN 1 ELSE 0 END ) +
  ( CASE WHEN table_2.field_jkl LIKE "%spork%" THEN 1 ELSE 0 END ) +
  ( CASE WHEN table_2.field_jkl LIKE "%pink%" THEN 1 ELSE 0 END )
)DESC;
于 2013-04-11T20:42:18.890 回答
2

因为 Bob 不存在于 table_1 元素中，并且 JOIN 会丢弃所有连接表中没有匹配行的行。

为了达到你想要的效果，在两个外观中将JOIN改为LEFT JOIN。这样，您将让所有“用户”元素与匹配的 table_1 和 table_2 元素连接，如果没有找到匹配项，则为 null。

于 2013-04-11T20:45:33.470 回答