数据库：
SQL
四大操作语句-
1.删 DELETE
DELETE FROM 表 WHERE 条件
2.增 INSERT
INSERT INTO 表 (字段列表) VALUES(值列表)
3.改 UPDATE
UPDATE 表 SET 字段=值,字段=值,... WHERE 条件
4.查 SELECT
SELECT * FROM 表 WHERE 条件
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子句：
WHERE 条件
WHERE name=‘blue‘
WHERE age>18
WHERE age<=18
WHERE age>=18 AND score<60
WHERE cach>100 OR score>10000
ORDER 排序
ORDER BY age ASC/DESC
ASC-升序(从小到大)
DESC-降序(从大到小)
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ORDER BY price ASC
*价格(price)升序排序，如果价格相同，再按销量(sales)降序排序
ORDER BY price ASC, sales DESC
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GROUP 聚类-合并相同
*统计每个班人数
ID class name
"1" "1" "小明"
"2" "2" "小红"
"3" "1" "小刚"
"4" "2" "小华"
"5" "3" "小强"
"6" "3" "小四"
"7" "1" "小刘"
"8" "1" "小花"
SELECT * FROM student_table;
ID class name
"1" "1" "小明"
"2" "2" "小红"
"3" "1" "小刚"
"4" "2" "小华"
"5" "3" "小强"
"6" "3" "小四"
"7" "1" "小刘"
"8" "1" "小花"
SELECT * FROM student_table GROUP BY class;
ID class name
"1" "1" "小明"
"2" "2" "小红"
"5" "3" "小强"
SELECT class FROM student_table GROUP BY class;
class
"1"
"2"
"3"
SELECT class,COUNT(class) FROM student_table GROUP BY class;
class COUNT(class)
1 4
2 2
3 2
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WHERE子句
ORDER子句——多条件排序
GROUP子句——合并
COUNT、MIN、MAX、AVG
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GROUP
