Sql 在将主键与 where 子句中的另一个值组合时返回空行

Question

I am getting empty row on following sql

SELECT * FROM flyers WHERE fId='6' AND 'userId'='400' 

MySQL returned an empty result set

(i.e. zero rows). (Query took 0.0004 sec)

But when i use

SELECT * FROM flyers WHERE fId='6'

Showing rows

0 - 0 (1 total, Query took 0.0005 sec).

As i got my result and that is correct using only primary key.

But i does not know why mysql returns empty row on using AND with primary key.

Note:- fId is flyer table primary key.

Answer 1

Try this:

1. Use back ticks for the column name not single quotes

2. If userid is int data type it means you have to remove single quotes userid=400

3. If fid is string it means fID='6'

   SELECT * FROM flyers WHERE fId =6 AND userId =400

Answer 2

In your query you have only one mistake, which is that you used single quotes around userId . Use back-ticks instead or nothing:

SELECT * FROM flyers WHERE fId='6' AND userId='400'  

SELECT * FROM flyers WHERE fId=6 AND userId=400// safe not to use quotes

But I suggest not to use quotes around numbers for below reason.

The real issue comes down to type casting. When you put numbers inside quotes, it is treated as a string and MySQL must convert it to a number before it can execute the query. While this may take a small amount of time, the real problems start to occur when MySQL doesn't do a good job of converting your string.

For example, MySQL will convert basic strings like '123' to the integer 123, but will convert some larger numbers, like ' 18015376320243459 ', to floating point. Since floating point can be rounded, your queries may return inconsistent results.