The example used before has data from multiple information modules that need to be displayed. Each module table has a title, id, and picture fields. In order to reduce multiple SQL queries, use union to combine the data from these tables into a result set and return it.

SELECT id,title,picture, 'A' AS module FROM A LIMIT 5
UNION 
SELECT id,title,picture, 'B' AS module FROM B LIMIT 5
UNION
SELECT id,title,picture, 'C' AS module FROM C LIMIT 5

For example, if you encounter a project with a large amount of data and need to divide it into tables. The same structure is divided into more than 100 tables, then when you query, you need to query each of these 100 tables and union all to obtain the data.

Tips:

select goods "商品", case goods when 1 then price then price*1.2 when 2 then price*1.5 else 0 end
from talbe

Disadvantage: poor code readability

select goods, price*1.2
from table where goods=1
union
select goods, price*1.5
from table where goods=2
union
select goods, 0
from table where goods not in (1, 2)

Sometimes union can be used to solve some strange judgment statements. For example, returning the total of the report together