How to realize multi-table union search?

the structure of the table is as follows:

now I need to query the contents from different tables according to the user"s input. How can I quickly and effectively find out the results I want?

my idea is to associate the three tables, use case.when to determine which table to take data from according to the value of flag, and finally form a temporary table, and then filter the data from the temporary table according to what the user enters.

are there any other more effective ideas for reference?

Mysql sql oracle sqlserver postgresql

May.06,2021

Table 1 is associated with Table An and Table B once, and the union, code is roughly as follows:

select ta.*
from t1 join ta on t1.id_a = ta.id
where t1.flag = 'a'
union all
select tb.*
from t1 join tb on t1.id_b = tb.id
where t1.flag = 'b'

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MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1bbaf47-30233.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1bbaf47-30233.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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