Question

假设我创建了3个表格，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");

然后我运行此查询...

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;

为什么它不会返回Bob的“spork”条目？您可以在http://sqlfiddle.com/#!2/cbbda/5

中查看此查询

不言而喻，这不是我的实际代码，但它足够相似，并且具有相同的问题，我将能够找出问题所在。

Answer 1

由于默认JOIN为INNER，因此查询不会返回表返回NULL的结果。使用LEFT JOIN会返回“Bob”行，包含NULL个值。

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

以下是各种JOIN类型的直观表示： 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;

Answer 2

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

为了实现您想要的效果，请在两个外观中通过LEFT JOIN更改JOIN。这样，您将使用table_1和table_2元素连接的所有“Users”元素匹配，如果未找到匹配项，则为null。

并非所有数据都来自mysql查询

2 个答案: