sqlite查询子句中需要括号 [英] Need of parentheses in sqlite query clause

问题描述

我正在运行以下 sqlite3 查询并注意到前两个查询没有给我预期的答案，即 1240969 + 1225691 != 1531026.如果我在 'or' 子句周围使用括号，我会得到预期的结果.这是为什么?

I am running the following sqlite3 queries and notice that the first two queries are not giving me the expected answer, i.e. 1240969 + 1225691 != 1531026. If I use parentheses around the 'or' clauses I get the expected result. Why is this?

sqlite> select count(*) from d where county = "A" or county = "D" and year = "1911";
1240969
sqlite> select count(*) from d where county = "A" or county = "D" and year = "1901";
1225691
sqlite> select count(*) from d where county = "A" or county = "D";
1531026
sqlite> select count(*) from d where (county = "A" or county = "D") and year = "1901";
748015
sqlite> select count(*) from d where (county = "A" or county = "D") and year = "1911";
783011

推荐答案

在 AND 和 OR 2 个操作符之间，AND 有优先权，所以这个:

Between the 2 operators AND and OR, AND has precedence, so this:

county = "A" or county = "D" and year = "1911"

评价像

county = "A" or (county = "D" and year = "1911")

所以它不一样:

(county = "A" or county = "D") and year = "1911"

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