Let t be a mysql table and n be an integer column in it. I would be interested if the following query can be modified so that the expression n - 10 is calculated only once.
设t是mysql表，n是其中的整数列。如果可以修改以下查询，我感兴趣的是，表达式n - 10只计算一次。
UPDATE t SET n = if(n - 10 < 1, 1, n - 10) WHERE n = 5;
The query does not make sense, I know, but I need to use the same pattern in the real application where values 10 and 5 would be php variables.
我知道，查询没有意义，但我需要在实际应用程序中使用相同的模式，其中值10和5是php变量。
2 个解决方案
#1
3
Allright, i have finally found the right syntax. The parentheses around @tmp := n - 10 are crucial.
好吧，我终于找到了正确的语法。@tmp:= n - 10的括号是关键。
UPDATE t SET n = if((@tmp := n - 10) < 1, 1, @tmp) WHERE n = 5;
#2
0
You can just do such logic in php:
你可以用php做这样的逻辑:
$value = $n - 10;
$value = $value < 1 ? 1 : $value;
$sql = "UPDATE t SET n = $value WHERE n = $n";
#1
3
Allright, i have finally found the right syntax. The parentheses around @tmp := n - 10 are crucial.
好吧，我终于找到了正确的语法。@tmp:= n - 10的括号是关键。
UPDATE t SET n = if((@tmp := n - 10) < 1, 1, @tmp) WHERE n = 5;
#2
0
You can just do such logic in php:
你可以用php做这样的逻辑:
$value = $n - 10;
$value = $value < 1 ? 1 : $value;
$sql = "UPDATE t SET n = $value WHERE n = $n";