Select from two tables and order by datetime stamps

Question

I have two MySQL tables similar to below:

MESSAGES_IN
messageID
receivedDate
message

MESSAGES_OUT
messageID
sentDate
message

Basically, I want to select messages from both tables and order them by their respective dates. For example, if the data set were:

MESSAGES_IN
1  2014-04-24 12:26:33  Test message inbound
2  2014-04-24 19:12:15  Another test message inbound

MESSAGES_OUT
1  2014-04-24 12:31:33  Test message outbound
2  2014-04-24 19:16:15  Another test message outbound

Then I want the output to be:

2014-04-24 12:26:33  Test message inbound
2014-04-24 12:31:33  Test message outbound
2014-04-24 19:12:15  Another test message inbound    
2014-04-24 19:16:15  Another test message outbound

I've attempted the query below but I'm getting error that my ORDER BY is an unknown column:

SELECT i.receivedDate, i.message
FROM MESSAGES_IN i
UNION
SELECT sentDate, o.message
FROM MESSAGES_OUT o
ORDER BY receivedDate, sentDate;

Thank you!

Answer 1

MySQL will use the column names from the first query when using UNION, so just use the first query's column name in the ORDER BY clause:

SELECT i.receivedDate, i.message
FROM MESSAGES_IN i
UNION ALL
SELECT o.sentDate, o.message
FROM MESSAGES_OUT o
ORDER BY receivedDate

It's common to use an alias like this to keep the column name meaningfull:

SELECT i.receivedDate AS sendreceiveDate, i.message
FROM MESSAGES_IN i
UNION ALL
SELECT o.sentDate, o.message
FROM MESSAGES_OUT o
ORDER BY sendreceiveDate

Also, you probably want UNION ALL to avoid losing duplicate rows in the results.

Answer 2

Since column names are different, you can use "ORDER BY 1" which means first column.

(SELECT i.receivedDate as messageDate, i.message
FROM MESSAGES_IN i )
UNION
( SELECT o.sentDate as messageDate, o.message
FROM MESSAGES_OUT o )
ORDER BY 1

I believe MySQL can use the alias in this case too, so you can replace "1" with messageDate.