MySQL performs an inner join on query. The query result is incorrect because the associated fields have different field types.

today, The test students reported that there was a problem with XX's query function , The unconfigured data is still found , Look through the code , Extract queried sql, Query the client to find whether there is any associated data or not , After checking for a long time, I found that , It is caused by mismatching of associated field types .

Solution ： Use mysql Of cast function , For example, the modification in the following picture can be solved .

OK, I know the solution , Explain the reason and the scenario , It is convenient for students to analyze their own situation .

First create 2 A watch ：

The place I enclose will be related to a fruit table

this 2 The enclosed parts of a table are used for association query , You can see that one is int(11)   One is varchar(64)   type ,2 What happens when two different types of associations are made ？ Let's go on to explain .

Enumerate 2 Data of tables ：

goods surface ：

orange surface ：

 

See the data of the two tables , Next I need goods Tabular id Fields and orange Tabular goods_type Fields are associated , Students who understand should be able to know , Theoretically, the data found after correlation must look like the following ：

But after we implemented , Below sql What data is generated after the statement ？

sql The statement is as follows ：

select g.name as goods_name ,o.goods_type,o.name
from goods g inner join orange o on g.id = o.goods_type
where g.type = 1

The queried data are as follows ：

EH ？ I found one goods_type yes 1ad...（ It's small anyway T Fabricate ）, This shouldn't be . Yes , I met this at first bug That's what I thought when I was , Later, many people inquired about it ,mysql When associating different types of fields , The field will be converted to the same type for comparison , This may lead to such uncorrelated data .

therefore , Students who encounter the above problems , Refer to the situation mentioned at the beginning of this post to solve .

But remember , When doing table Association , The associated fields must be the same , Because if you do large data linked table query , This is because the query efficiency is reduced due to linked tables , If the field types are different , And it also causes mysql The conversion type comparison will be carried out continuously , In this case , It may reduce the query efficiency , Infinite heap of slow queries sql But it will cause problems for the whole system .