核心语法:SELECT COUNT(*) FROM 表名;(统计全表总行数,最常用、性能最优)SELECT COUNT(字段名) FROM 表名;(统计该字段非空数据,排除NULL)案例:统计user这张表的总行数:SELECT COUNT(*) FROM user;
2. 且逻辑(AND):俩条件都满足
核心语法:SELECT COUNT(*) FROM 表名 WHERE 条件1 AND 条件2;案例:统计“男且北京”:SELECT COUNT(*) FROM user WHERE gender = '男' AND city = '北京';
3. 或逻辑(OR):俩条件二选一
核心语法:SELECT COUNT(*) FROM 表名 WHERE 条件1 OR 条件2;案例:统计“男或北京”:SELECT COUNT(*) FROM user WHERE gender = '男' OR city = '北京';
4. 多条件或:用IN更省力
核心语法:SELECT COUNT(*) FROM 表名 WHERE 字段 IN(值1, 值2, 值3);案例:统计“北上广”:SELECT COUNT(*) FROM user WHERE city IN('北京','上海','广州');
5. 必避坑:且+或混合,必加括号
写法1:SELECT COUNT(*) FROM user WHERE gender='男' AND city='北京' OR city='上海';统计北京男的+上海的写法2:SELECT COUNT(*) FROM user WHERE gender = '男' AND (city = '北京' OR city = '上海');统计男的,北京或者上海的划重点:逻辑是关键