如何在一个MySQL查询中获取另一个表中引用的数据

Question

I would to select some data from mysql. However, some of the data stored in the table I am querying from are in codes and to get the text description I need to reference that data to another table.

TABLE: persons

SELECT id, first_name, last_name, address_code, customer_type_code
FROM persons
WHERE id = 1001    

TABLE: ref_address 

SELECT address_name FROM ref_address
WHERE address_code = 123


TABLE: ref_customer_type_code   

SELECT customer_type_name FROM ref_customer_type_code
WHERE customer_type_code = 456

How can I combine all three queries together to return id, first_name, last_name, address_name, customer_type_name in one query instead of querying them 3 times like this?

Answer 1

What you're looking for is a JOIN .

In a JOIN , you specify two tables and how they are related to one another. In a single SELECT statement, you can have multiple JOIN clauses.

SELECT
    p.id, p.first_name, p.last_name, p.address_code, p.customer_type_code,
    a.address_name,
    t.customer_type_name

FROM
    persons p

    JOIN ref_address a
        ON p.address_code = a.address_code

    JOIN ref_customer_type_code t
        ON p.customer_type_code = t.customer_type_code

WHERE
    p.id = 1001

This query says that the table persons and ref_address should be linked, or "joined", by the related columns address_code which are available in each table. Same goes with the tables persons and ref_customer_type_code being linked by the columns customer_type_code .

Answer 2

Please read the reference manual for join .

In short, you need to define a relation between your tables (I use aliases just to make things a bit "cheaper" to write):

select p.id, p.first_name, p.last_name, p.address_code, p.customer_type_code
     , ra.address_name
     , rctc.customer_type_name
from persons as p
     -- Join the persons table with the ref_address table, 
     -- using the address_code column of each table
     inner join ref_adress as ra 
                on p.address_code = ra.address_code
     -- Join the persons table with the ref_customer_type_code table
     -- using the customer_type_code column of each table
     inner join ref_customer_type_code as rctc 
                on p.customer_type_code = rctc.customer_type_code
where p.id = 1001

Notice that when you use multiple tables in a query it may be useful to define aliases to avoid having to write again and again the full name of the table. Also, it may be a good idea to explicitly specify each field's source table (by alias, if you are using it)