查询关键词
快速查找SQL关键词和语法说明
SELECT
用于从数据库表中查询数据
SELECT column1, column2 FROM table_name;
FROM
指定要查询的数据表
SELECT * FROM students;
WHERE
用于过滤查询结果，指定查询条件
SELECT * FROM students WHERE age > 20;
ORDER BY
对查询结果进行排序
SELECT * FROM students ORDER BY age DESC;
GROUP BY
按指定列对数据进行分组
SELECT grade, COUNT(*) FROM students GROUP BY grade;
HAVING
对分组后的结果进行过滤
SELECT grade, AVG(score) FROM students GROUP BY grade HAVING AVG(score) > 80;
JOIN
用于连接两个或多个表
SELECT * FROM students s JOIN classes c ON s.class_id = c.id;
DISTINCT
去除查询结果中的重复记录
SELECT DISTINCT grade FROM students;
LIMIT
限制返回的记录数量
SELECT * FROM students LIMIT 10;
COUNT
统计记录数量
SELECT COUNT(*) FROM students;
SUM
计算数值列的总和
SELECT SUM(score) FROM exam_results;
AVG
计算数值列的平均值
SELECT AVG(score) FROM exam_results;
MAX
获取列的最大值
SELECT MAX(score) FROM exam_results;
MIN
获取列的最小值
SELECT MIN(score) FROM exam_results;
LIKE
用于模糊查询，支持通配符
SELECT * FROM students WHERE name LIKE '张%';
IN
判断值是否在指定列表中
SELECT * FROM students WHERE grade IN ('大一', '大二');
BETWEEN
查询在指定范围内的数据
SELECT * FROM students WHERE age BETWEEN 18 AND 22;
IS NULL
判断字段是否为空
SELECT * FROM students WHERE email IS NULL;
AS
为列或表指定别名
SELECT name AS student_name FROM students;
CASE WHEN
条件判断，根据不同条件返回不同值
SELECT name, CASE WHEN score >= 60 THEN '及格' ELSE '不及格' END FROM students;
UNION
合并多个查询结果，自动去重
SELECT name FROM students UNION SELECT name FROM teachers;
INSERT
向表中插入新记录
INSERT INTO students (name, age) VALUES ('张三', 20);
UPDATE
更新表中的记录
UPDATE students SET age = 21 WHERE id = 1;
DELETE
删除表中的记录
DELETE FROM students WHERE id = 1;