Help needed for a sql query

I am just posting a simple scenario of my problem which is typical then thsi actually.

there are 2 tables PERSON & CUSTOMER

PERSON

FNAME LNAME SYNC
A B N
A C N
D E N
F G N
H I N
J K N

CUSTOMER

FNAME LNAME
A B
D E
H I
Now i have to update SYNC field in PERSON as Y for the records which exists in table customer, for eg these rescords exist in person table also
FNAME LNAME
A B
D E
H I

i want to make SYNC field as Y for these records in PERSON table.

I tried this way i wrote this query :

SELECT FNAME FROM PRAGS.CUSTOMER WHERE (FNAME) IN (SELECT FNAME FROM PRAGS.PERSON)

this will tell us if a particular FNAME exist in customer table or not, now to update the SYNC field in PERSON i tried using update query as

UPDATE PRAGS.PERSON SET SYNC='Y' WHERE EXISTS(SELECT FNAME FROM PRAGS.CUSTOMER WHERE (FNAME) IN (SELECT FNAME FROM PRAGS.PERSON))

this updates all the SYNC as Y because as asoon as SELECT FNAME FROM PRAGS.CUSTOMER WHERE (FNAME) IN (SELECT FNAME FROM PRAGS.PERSON) evalutaes to be truew it updates all sync as Y. i also tried and statemnet but it didn't work. please suggest me a way to work it out

Also this is just a small example of what i want to do. Actual person table and customer table will contain more than 25 lac records. so how to do it optimally.

i want to do it using SQL query.

i have also done this using a java program below, this solved my program but i dont know how to do it optimally because recordset can not hold 25 lac records , thsi is not viable.

String str = "SELECT FNAME FROM PRAGS.PERSON";

rs = stmt.executeQuery(str);

while(rs.next()){
String cfName="";
String fName = rs.getString(1);
//String lName = rs.getString(2);

//rs1 = stmt.executeQuery("SELECT FNAME, LNAME FROM PRAGS.CUSTOMER WHERE (FNAME, LNAME) IN ("+fName+","+lName+")");
rs1 = stmt1.executeQuery("SELECT FNAME FROM PRAGS.CUSTOMER WHERE (FNAME) IN ('"+fName+"')");
while(rs1.next()){
cfName = rs1.getString(1);
}

if(!cfName.equals("")){
stmt1.executeUpdate("UPDATE PRAGS.PERSON SET SYNC = 'Y' WHERE FNAME ='"+fName+"'");
}

try {
if(rs1!=null){
rs1.close();
}
} catch (SQLException e) {
System.out.println("Exception is "+e.getMessage());

}

}

please help
thanks.

Which rdbms?

cprash.aggarwal, since you don't mention what DBMS or version you are using, I don't know if this will work for you or not. It does work in the DBMS I am familiar with:

UPDATE PRAGS.PERSON P
SET SYNC = 'Y'
WHERE EXISTS (SELECT *
FROM PRAGS.CUSTOMER C
WHERE A.FNAME = C.FNAME
AND A.LNAME = C.LNAME)