选择(Selection):根据条件对数据进行筛选
SELECT * FROM students WHERE sex = 'female';
投影(Projection):指从表中选取特定列或计算新的虚拟列。
SELECT 学号, 姓名, 总学分 FROM students;
MySQL连接(Join):将两个或多个表按照共同字段关联起来获得更丰富的结果集。常见的连接类型有内连接(INNER JOIN)、左外连接(LEFT OUTER JOIN)、右外连接(RIGHT OUTER JOIN)和全外连接(FULL OUTER JOIN)。
SELECT t1.column_name1, t2.column_name2 FROM table1 AS t1 INNER JOIN table2 AS t2 ON t1.common_field = t2.common_field;