sqlite查询子句中需要括号

Question

我正在运行以下sqlite3查询，请注意前两个查询没有给我期望的答案，即1240969 + 1225691！=1531026。如果在“或”子句周围使用括号，则会得到预期的结果。为什么会这样？

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

Answer 1

在两个运算符AND和OR之间，AND具有优先权，因此：

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

评估类似

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

因此与以下内容不同：

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