I have opened a demat & trading account with sbi but want to open trading account only with zerodha and keeping stocks in SBI demat and selling that stocks from zerodha

you can open zerodha trading account and link your sbi DP with zerodha trading account, you can buy the delivery stocks from zerodha trading account , and zerodha will deliver stocks to sbi DP, but while selling you have to sell it from sbi only. (as holdings are with sbi DP)