无法在mysql中将值从一个表更新到另一表

Question

Ledgers Table

ledger_id   ledger_name dep_id  dr_bal   cr_bal
   1          Purchase     2     NUll     NUll

Transaction Table

trans_id    trans_date  ledger_id   ledger_name    amount   trans_type
1            3/2/2004      1        Purchase A/C    84500   dr
2            3/12/2004     6         Cash A/C       20000   cr 

These are my tables, Ledgers and Transactions . I want to update ledgers.dr_bal from transactions.amount , based on ledger_id which is the primary key in ledgers table.

Want to copy the values from transactions.amount to dr_bal based on trans_type ='dr'

So far I have tried doing ,

UPDATE ledgers
SET dr_bal =(select sum(If(tbl_transactions.trans_type = 'dr' AND transactions.ledger_id = 1), amount,0) FROM transactions)
where ledgers.ledger_id =1;

But am unable to run the above query, as it throws an error at the Where clause at the end.

Have tried looking into various questions related to updating tables here. But am really stuck.

Answer 1

Try this query!

UPDATE ledgers
        LEFT JOIN
    (SELECT 
        SUM(amount) soa, ledger_id
    FROM
        tbl_transactions
    WHERE
        tbl_transactions.trans_type = 'dr'
            AND tbl_transactions.ledger_id = 1) t ON (ledgers.ledger_id = t.ledger_id) 
SET 
    ledgers.dr_bal = coalesce(t.soa, 0);

If you would like to update all ledgers with the transactions amount, remove the condition of tbl_transactions.ledger_id = 1 and introduce GROUP BY tbl_transactions.ledger_id in the sub-query.