Trouble in MySQL querying -


i developing digital mobile loyalty app , have face problem in querying mysql code...

i have 4 tables: customer, card, redemption, merchant

currently trying want system first check if customer possess loyalty card of merchant when making redemption

  • if yes, can proceed
  • if no, system create card them , proceed

the redemption code generated merchant...

the problem how should query code so?

customer   - customer_id  - c_email  - c_password  - c_name  card   - card_id  - chop_amt  - customer_id*  - merchant_id*  merchant   - merchant_id  - merchant_name  redemption   - red_id  - red_code  - card_id*  - merchant_id*

i tried writing code own now...can some1 please me check?

select *  customer customer  join card card    on customer.customer_id = card.customer_id  join redemption redemption    on card.merchant_id = redemption.merchant_id  card.merchant_id = redemption.merchant_id ,       redemption.red_code = 'a002'

i recommend putting unique index on card table merchant / customer entries. if go approach, can insert ... on duplicate key update id = id. ensure record exists regardless.

to create index, this

create unique index customer_merchant on card (customer_id, merchant_id);

now have in place, go ahead , insert, not insert new row if 1 exists

insert card (chop_amt, customer_id, merchant_id) values(0.00, 1, 1) on duplicate key update card_id = card_id

the update card_id = card_id part alias "don't anything", it's setting card_id itself


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