How to use three tables to perform FULL OUTER JOIN operation in MySQL?
P粉852114752
2023-09-01 21:48:57
<p>Consider the following table:</p>
<pre class="lang-sql prettyprint-override"><code>create table `t1` (
`date` date,
`value` int
);
create table `t2` (
`date` date,
`value` int
);
create table `t3` (
`date` date,
`value` int
);
insert into `t1` (`date`, `value`)
values ("2022-01-01", 1),
("2022-03-01", 3),
("2022-04-01", 4);
insert into `t2` (`date`, `value`)
values ("2022-01-01", 1),
("2022-02-01", 2),
("2022-04-01", 4);
insert into `t3` (`date`, `value`)
values ("2022-01-01", 1),
("2022-02-01", 2),
("2022-03-01", 3);
</code></pre>
<p><code>t1</code>The table is missing<code>2022-02-01</code>date,<code>t2</code>is missing<code>2022-03-01< /code>, <code>t3</code> is missing <code>2022-04-01</code>. I want to join these three tables to produce the following result: </p>
<pre class="brush:php;toolbar:false;">| t1.date | t1.value | t2.date | t2.value | t3.date | t3.value |
| | | | | | |
| 2022-01-01 | 1 | 2022-01-01 | 1 | 2022-01-01 | 1 |
| null | null | 2022-02-01 | 2 | 2022-02-01 | 2 |
| 2022-03-01 | 3 | null | null | 2022-03-01 | 3 |
| 2022-04-01 | 4 | 2022-04-01 | 4 | null | null |</pre>
<p>I know how to do a <code>full outer join</code> between two tables, but between three or more tables it's more complicated. I tried a query like this but it didn't produce the results I wanted: </p>
<pre class="lang-sql prettyprint-override"><code>select *
from `t1`
left join `t2` on `t2`.`date` = `t1`.`date`
left join `t3` on `t3`.`date` = `t2`.`date` or `t3`.`date` = `t1`.`date`
union
select *
from `t1`
right join `t2` on `t2`.`date` = `t1`.`date`
right join `t3` on `t3`.`date` = `t2`.`date` or `t3`.`date` = `t1`.`date`;
</code></pre></p>
You are on the right track, but you have to consider all combinations - 3 tables - 4 union clauses.