According to a certain condition, query "
Yes " and "No" from the database table, and there are only two states, so why do you need to select count(*) when writing SQL? Whether it is a new star programmer or a superb The programmer Lao Bai who has been on the battlefield for many years has the same count as always
At present, most people's writing method
has repeatedly reviewed the code, and found the phenomenon as it is: In the business code, it is necessary to query whether there are records based on one or more conditions, and do not care how many records there are. Common SQL and code writing are as follows
##### SQL写法:
SELECT count(*) FROM table WHERE a = 1 AND b = 2
##### Java写法:
int nums = xxDao.countXxxxByXxx(params);
if ( nums > 0 ) {
//当存在时，执行这里的代码
} else {
//当不存在时，执行这里的代码
}
Does it feel OK? There is no problem to
optimize the solution. The
recommended writing is as follows:
##### SQL写法:
SELECT 1 FROM table WHERE a = 1 AND b = 2 LIMIT 1
##### Java写法:
Integer exist = xxDao.existXxxxByXxx(params);
if ( exist != NULL ) {
//当存在时，执行这里的代码
} else {
//当不存在时，执行这里的代码
}
SQL no longer uses count, but instead uses LIMIT 1, so that when the database query encounters one, it will return. Don't continue to find how many more ones are left. Just judge whether it is not empty in the business code.
Summary
The more the number of items found according to the query conditions, the more obvious the performance improvement, and in some cases, the creation of joint indexes can be reduced.
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