mysql - Finding whether a row exists in mysql or not using SELECT EXISTS

Question

If there exists a row in the table FINES with the required value then I just want to update the fields Else I want to add a new row in the table.

if (mysql_query("SELECT EXISTS(SELECT * FROM fines WHERE SId='$sid' AND Semester='$sem')")>=1)

 mysql_query("UPDATE fines SET fines.TFine=fines.TFine+'$fine' WHERE SId='$sid' AND Semester='$sem'");           

else

   mysql_query("INSERT INTO fines(SId, Semester, TFine, Flag) VAlUES('$sid','$sem', '$fine', 0)");

But the if condition is not working!! Please help

Answer 1

INSERT ... ON DUPLICATE KEY UPDATE如果记录已经存在，则使用 MySQL更新记录，如果不存在则插入。

您需要做的第一件事是unique在一个或多个列上指定一个约束，

ALTER TABLE fines ADD CONSTRAINT tb_uq UNIQUE (SId, Semester)

一旦你执行了语句，你可以简单地创建一个这样的语句，

INSERT INTO fines(SId, Semester, TFine, Flag) 
VALUES ('$sid','$sem', '$fine', 0)
ON DUPLICATE KEY UPDATE TFine = TFine +  '$fine';

• INSERT ... ON DUPLICATE KEY UPDATE 语法