The used SELECT statements have a different number of columns

Question

For examples I don't know how many rows in each table are and I try to do like this:

SELECT * FROM members 
UNION 
SELECT * FROM inventory

What can I put to the second SELECT instead of * to remove this error without adding NULL's?

Purveyor

Why would you want to union two tables of completely different contexts? – Howund

You shoud never use select * in any query that will run on production. You should always specify only the columns you need both for maintainibility and performance. – Exemplary

Answer 1

Put the columns names explicitly rather than *, and make sure the number of columns and data types match for the same column in each select.

Update:

I really don't think you want to be UNIONing those tables, based on the tables names. They don't seem to contain related data. If you post your schema and describe what you are trying to achieve it is likely we can provide better help.

Albania

I know nothing about any names, nothing. Is it still possible? – Purveyor

@hey: Use DESC your_table_name to find out the columns and their data types before writing the UNION query. – Capri

@hey: Umm... sorta non-sequitor there... if you type the column names in manually, you will reduce the risk of SQL injection. – Dys

@Frus SQL injection!?!?!? What in tarnation are you talking about? SELECT * vs. SELECT Col1, Col2 has absolutely nothing to do with SQL injection. – Hither

@Emtucifor: It was hey that brought up the topic of SQL injection. My guess is that hey thought SELECT * could lead to SQL injection. I used to have a manager who would also worry about that so insisted that column names be always be explicit. It didn't make much sense to me, but it felt like that guy and hey took the same classes in school... – Dys

@Frus oops, I meant to address that to hey, sorry about that. Let me try again. – Hither

@hey SQL injection!?!?!? What in tarnation are you talking about? SELECT * vs. SELECT Col1, Col2 has absolutely nothing to do with SQL injection. – Hither

Answer 2

you could do

SELECT *
from members
UNION
SELECT inventory.*, 'dummy1' AS membersCol1, 'dummy2' AS membersCol2
from inventory;

Where membersCol1, membersCol12, etc... are the names of columns from members that are not in inventory. That way both queries in the union will have the same columns (Assuming that all the columns in inventory are the same as in members which seems very strange to me... but hey, it's your schema).

UPDATE:

As HLGEM pointed out, this will only work if inventory has columns with the same names as members, and in the same order. Naming all the columns explicitly is the best idea, but since I don't know the names I can't exactly do that. If I did, it might look something like this:

SELECT id, name, member_role, member_type
from members
UNION
SELECT id, name, '(dummy for union)' AS member_role, '(dummy for union)' AS member_type
from inventory;

I don't like using NULL for dummy values because then it's not always clear which part of the union a record came from - using 'dummy' makes it clear that the record is from the part of the union that didn't have that record (though sometimes this might not matter). The very idea of unioning these two tables seems very strange to me because I very much doubt they'd have more than 1 or 2 columns with the same name, but you asked the question in such a way that I imagine in your scenario this somehow makes sense.

Dys

Why not use NULL instead of 'dummy1' and 'dummy2'? – Heterotrophic

Even if this would work, you still need to specify column names as the columns may or may not be in the same order on both tables (or stay in the same order forever). Plus if inventory then had anopther column added, the union would break. Never do a UNOIN without specifying the columns. Also all unions should be considered as candiates fro UNION ALL which is much more performant if there is no possiblity of dupes that need to be filtered out. – Exemplary

Telling how to do something that is so obviously incorrect is hurting the poor chap asking the question, not helping him. – Hither

@Emtucifor: Considering his question barely made sense (union two completely different tables), I don't really see what's wrong with this answer. It would work for what he wants to do. WHY he wants to do that is something else entirely. – Dys

IF you use Null for the dummy values you wil probably want to cast it to the correct datatype. Null is considered an int if not specifially cast to something else. – Exemplary

Answer 3

Are you sure you don't want a join instead? It is unlikely that UNOIN will give you what you want given the table names.

Exemplary

Answer 4

Try this

(SELECT * FROM members) ;
(SELECT * FROM inventory);

Just add semicolons after both the select statements and don't use union or anything else. This solved my error.

Henley

This is slightly different than a SQL statement with UNION in it which returns one result combined from two tables whereas this returns two results. In most cases this needs to be handled differently on the client side and thus cannot be used as a direct replacement of UNION. In addition, the parentheses around the SQL aren't needed. – Willywilly

Answer 5

I don't know how many rows in each table

Are you sure this isn't what you want?

SELECT 'members' AS TableName, Count(*) AS Cnt FROM members 
UNION ALL
SELECT 'inventory', Count(*) FROM inventory

Hither

Answer 6

Each SELECT statement within the MySQL UNION ALL operator must have the same number of fields in the result sets with similar data types Visit https://www.techonthenet.com/mysql/union_all.php

Peppie

Answer 7

I got the same error below:

ERROR 1222 (21000): The used SELECT statements have a different number of columns

Because I tried to store two values 1 and John of id and name columns into only one @result which is a user-defined session variable as shown below:

mysql> SELECT * FROM person;
+----+-------+
| id | name  |
+----+-------+
|  1 | John  |
|  2 | David |
+----+-------+
...
mysql> SELECT id, name INTO @result FROM person WHERE id = 1; -- Error

But, I could store two values 1 and John of id and name columns into @result1 and @result2 respectively without error as shown below:

mysql> SELECT id, name INTO @result1, @result2 FROM person WHERE id = 1;
...
mysql> SELECT @result1, @result2; 
+----------+----------+
| @result1 | @result2 |
+----------+----------+
|        1 | John     |
+----------+----------+

Garett