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SELECT
transaction.entityid,
transaction.tranid,
transaction.job, customrecord_recordtype.custrecord_customfield

FROM transaction
LEFT JOIN transaction ON
		( transaction.job = customrecord_recordtype.custrecord_customfield)


WHERE transaction.id = '12345'

Use table aliases for the 2 instances of `transaction` in the from clause

Or maybe your join is a mistake and you meant to left join on a different table 

At first glance I thought you were doing a self-join, but I think you just made a mistake and need to change your join to use LEFT JOIN customrecord_recordtype instead of transaction 

Getting better, now it thinks that 'job' (the project number) on the transaction doesn't exist

SELECT
t.entity,
t.tranid,
c.custrecord_customfield

FROM transaction as t
LEFT JOIN customrecord as c on
		( t.job = c.custrecord_custom_project_fld )


WHERE t.id = '6075760'