MySQL does not support FULL OUTER JOIN, and can be implemented through LEFT JOIN and RIGHT JOIN combined with UNION. 1. Use LEFT JOIN and RIGHT JOIN joint query, merge and deduplication through UNION, pay attention to the consistent order of the fields; 2. Use COALESCE to unify the primary key when processing duplicate data to facilitate subsequent processing; 3. For complex scenarios, temporary tables or subqueries can be used to process the left and right table data separately and then merge them to improve readability. The core is to merge left and right results and remove heavy weights.
MySQL itself does not support FULL OUTER JOIN , but in actual business we often need to get all records of the two tables, regardless of whether there is a match or not. At this time, you have to simulate this effect in other ways.
In fact, the core idea is: merge the results of the left connection and the right connection , and remove duplicate data.
Let’s take a look at several commonly used implementation methods.
Joint query using LEFT JOIN and RIGHT JOIN
This is the most common and easy to understand practice. First, use LEFT JOIN to obtain all records on the left table, then use RIGHT JOIN to fill in the part that has the right table but does not have the left table, and finally use UNION to merge and remove the heavy load.
SELECT * FROM left_table LEFT JOIN right_table ON left_table.id = right_table.lid UNION SELECT * FROM left_table RIGHT JOIN right_table ON left_table.id = right_table.lid;
- Note that the field order must be consistent, otherwise UNION will error.
- If you only care about some fields, you can explicitly list the field names to avoid full table scanning.
-
UNIONwill automatically deduplicate. If duplicate data is allowed,UNION ALLcan be used to improve performance.
Use COALESCE to locate primary keys when processing duplicate data
In the FULL OUTER JOIN scenario, both sides may be empty. To unify primary keys, COALESCE is usually used to specify a "representative column".
for example:
SELECT
COALESCE(l.id, r.lid) AS id,
l.name AS left_name,
r.detail AS right_detail
FROM left_table l
LEFT JOIN right_table r ON l.id = r.lid
UNION
SELECT
COALESCE(l.id, r.lid),
l.name,
r.detail
FROM left_table l
RIGHT JOIN right_table r ON l.id = r.lid;The benefits of doing this are:
- The primary key is unified, which facilitates subsequent processing.
- You can clearly see which side has a value and which side is NULL.
Simplify logic with temporary tables or subqueries (suitable for complex scenarios)
If your SQL is already complicated or involves multiple correlation conditions, you can consider finding out the results of the left and right parts separately and then doing joint operations.
For example:
SELECT * FROM (
SELECT id, name, NULL AS detail FROM left_table
) AS left_side
UNION
SELECT * FROM (
SELECT lid AS id, NULL AS name, detail FROM right_table
) AS right_side;This method is suitable for:
- Query structure and inconsistent fields.
- Data logic from different sources needs to be processed separately.
The disadvantage is that it is a bit verbose to write, but it is better readable.
In general, the key to simulate FULL OUTER JOIN is to merge the LEFT and RIGHT results, paying attention to the fields corresponding to deduplication. Although MySQL does not have native support, it can be done with UNION plus LEFT/RIGHT JOIN. Basically, just choose one that suits your scenario.
The above is the detailed content of how to simulate full outer join in mysql. For more information, please follow other related articles on the PHP Chinese website!
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