Gbase8s database for update clause

When you plan to update prepared by SELECT The value returned by the statement , When accessing these values , Please be there. ESQL/C Application and in

DB-Access Use in FOR UPDATE Clause .

Preparation includes FRO UPDATE Clause SELECT sentence , Equivalent to preparing without FOR UPDATE Clause SELECT language

sentence , Then declare for the prepared statement FOR UPDATE The cursor .

FOR UPDATE Keyword notifies the database server of possible updates , Cause it to use more than accompanying Select Cursor is more strict

lock . Without this clause , You cannot modify data through a cursor . You can specify which columns can be updated .

When you are SELECT . . . FOR UPDATE Statement after declaring a cursor , You can use the with WHERE CURRENT OF Clause

UPDATE or DELETE Statement to update or delete the currently selected row . keyword CURRENT OF Reference the most recently accessed row ;

They replace in WHERE The usual conditional expression in clause . To update a row with a specific value , Your program may include the following

The statement in the column example ：

EXEC SQL BEGIN DECLARE SECTION;

char fname[ 16];

char lname[ 16];

EXEC SQL END DECLARE SECTION;

. . .

EXEC SQL connect to ‘stores_demo’;

/* select statement being prepared contains a for update clause */

EXEC SQL prepare x from ‘select fname, lname from customer for update’;

EXEC SQL declare xc cursor for x;

for (;

{

EXEC SQL fetch xc into $fname, $lname;

if (strncmp(SQLSTATE, ‘00’, 2) != 0) break;

printf(“%d %s %s\n”,cnum, fname, lname );

if (cnum == 999) --update rows with 999 customer_num

EXEC SQL update customer set fname = ‘rosey’ where current of xc;

}

EXEC SQL close xc;

EXEC SQL disconnect current;

SELECT . . . FOR UPDATE sentence , image Update Cursor like , Allows you to perform those individual uses UPDATE sentence

Updates that cannot be performed , Because the decision to update and the value of the new data item can be based on the original content of the row .UPDATE

Statement cannot query the table being updated .

notes ： In the cursor FETCH A normal update within the loop is not guaranteed in UPDATE Then access the updated row again .

WHERE CURRENT OF The specification will UPDATE Contact Update The cursor , And make sure that each row is updated only once , By inside

Keep a list of rows that have been updated . These lines will not be Update The cursor accesses again .