Question

I was thinking about implementing a state table in MySQL for several Bash scripts to poll. The idea is that script #n is cleared to run once script #n-1 has finished running and has written its state and status into the state table in a MySQL db. Basically the state table would look like:

+----+-------+--------+
| id | state | status |
+----+-------+--------+
| 44 |     2 |   OK   |
| 45 |     3 |   OK   |
| .. |   ... |  ...   |
| 55 |     2 |   OK   |
+----+-------+--------+

The id column is auto_increment so that can be used to define the last row.

At the moment I’ve come up with:

#!/bin/bash
#...
VARIABLE=$(mysql mydbname --skip-column-names -s --local -e \
         "SELECT count(*) \
          FROM statetable \
          WHERE state=2 AND \
                status='OK' AND \
                id=(SELECT MAX(id) FROM statetable)")
if [ $VARIABLE -eq 1 ] ; then #...

Is there a right way to check if the last row in the table has the wanted state value and status pair (2 and OK in the example above) without a subquery and to pass that information to the Bash script?

ADDED SOLUTION:

#!/bin/bash
function check_state () {
    VARIABLE=$(mysql mydbname --skip-column-names -s --local -e \
             "SELECT (state=2) && (status='OK') \
              FROM statetable \
              ORDER BY id DESC \
              LIMIT 1;")
    return $VARIABLE
}

if ! check_state ; then #...

Answer 1

In MySQL, you can select not only fields, but also expressions and operations on them. Somewhat surprisingly this works for logical tests as well, the kind we use, for example in a where clause. That means that you could use the following SQL to get a boolean (0 / 1) result:

select (state = 2) && (status = 'OK')
from statetable
order by id desc
limit 1

You can try it out on this SQL Fiddle.

Sorry that I did not understand the question at first, but as it seems everybody did 🙂

The query you have written will most likely always return a value of 1, because it does not select the maximum id, it selects the number of rows with a maximum id, and if there is at least one row in the table, there is such a thing as a maximum id 🙂

Assuming that you need the last ID, a better way (without a subquery) would be to order the data, and just take the first row, something like:

SELECT id FROM statetable 
WHERE state=2 
  AND status='OK' 
ORDER BY ID DESC
LIMIT 1