SQL查询从表1和表2中选择？

Question

I'm struggling to find an answer to a query I want to run. I want to

SELECT * FROM users AND tempusers WHERE username='joe'

Both tables have a column named username. I keep getting hit with SQL 1064 syntax error, which isn't very helpful to me.

DB structure:

DB Name : mydb

Table : users Columns : id, username, email, password.

Table : tempusers Columns : id, username, email, password, activationcode.

Answer 1

The error you're getting is because you can't select from table1 AND table2 ; that's not usually valid SQL syntax. (I say "usually" because MySQL has some proprietary syntax that most other databases don't use.)

It doesn't appear that a JOIN will work, because you're not describing that sort of relation between the tables.

This should work if both tables have exactly the same structure (column names and types and the same number of columns):

SELECT * FROM users WHERE username = 'joe'
UNION ALL
SELECT * FROM tempusers WHERE username = 'joe'

If the tables don't have the same exact schema, you'll have to name the columns in at least the first table SELECT :

SELECT 
  username, id, username, email, password, '' as activationcode
FROM 
  users 
WHERE 
  username = 'joe'
UNION ALL
SELECT 
  username, id, username, email, password, activationcode
FROM 
  tempusers
WHERE 
  username = 'joe'

You may have to provide blank spaces in place of '' for the activationcode in the first select, or use a value of the proper type for the activationcode column if it's not a character column (for instance, 0 for an integer column).

Answer 2

Potentially this will resolve the error and help you to think about why the error is occuring:

SELECT * 
FROM   users, tempusers 
WHERE  users.username='joe' 
       or tempusers.username='joe'

However - I strongly recommend using a proper join here. This query will be slow, and cumbersome. Ken White's answer is bang on (and should definitely be the accepted one) - I'm only posting this to help you to understand the initial error more clearly (the dbms is unable to differentiate which table's username column you are referring to and can't refer to both in one statement).