Magical Union

• This keyword cannot be said to have not been used , But it is only to query the same data , When different filtering conditions are required , Use union all Splice and then output together to view
• 8 month 24 Nikkei friends recommend an interview （ Hang up ）, Asked about the use of the keyword , I'm confused , So I came down to study , It is found that this keyword can really be used in practical work

describe

UNION Operators are used to connect more than two SELECT The results of a statement are combined into a set of results . That is to find the union of multiple query results .

union yes union distinct Abbreviation , It has the function of weight removal , And the default will be based on ASCII Sort .union all Don't deal with it .

Example

select product.hospital_id,1 tables,null field1,product.product_type
from db_product.tb_discount_product product
where product.spu_id = 258933
union all
select cart.pid,2,cart.status,null
from db_order.`tb_shopping_cart` cart
where cart.id = 174
order by tables desc;

Be careful

Same number of fields

Spliced multiple select The number of fields queried in the statement must be the same , As in my example above , Two select The number of fields queried in the statement is four , If it's not the same , The execution will report an error

Field names begin with the first select The query field shall prevail

As the example above , The fields of the query result are hospital_id,tables,field1,product_type These four fields .

Application scenarios

Multiple query results , Some fields have the same meaning , Want to splice together to see . This is the main requirement .

Pain points

But again, we may need to look at some other fields , However, these fields may only be included in one query and not in the other query .

union Provides a solution to this problem , Allow users to add query Columns . As the example shows tables Column 、field1 Column sum product_type Column .tables Column is used to distinguish which query statement the data comes from ;field1 The column in the first query does not , Only include... In the second query , A default value can be given in the first query null, In the second query, the specific value is queried ;product_type Then in field1 contrary .