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闸北区网站设计与制,怎么做好网络销售技巧,mianf网站,做网站的首页下拉列表一、sql基础语句1.select语句SELECT column1, column2, ... FROM table_name WHERE condition;其中#xff0c;SELECT关键字用于指定要查询的列#xff0c;可以使用*代表所有列#xff1b;FROM关键字用于指定要查询的表#xff1b;WHERE关键字用于指定查询的条件。select *…一、sql基础语句1.select语句SELECT column1, column2, ... FROM table_name WHERE condition;其中SELECT关键字用于指定要查询的列可以使用*代表所有列FROM关键字用于指定要查询的表WHERE关键字用于指定查询的条件。select * from student where sex’男’;2.insert语句INSERT INTO table_name (column1, column2, column3, ...)VALUES (value1, value2, value3, ...);其中table_name是要插入数据的表名column1、column2等是表中的列名value1、value2等是要插入的值。3.update语句UPDATE table_nameSET column1 value1, column2 value2, ...WHERE some_column some_value;其中table_name是要更新数据的表名column1、column2等是要修改的列名value1、value2等是要修改的值WHERE子句是用于指定要更新的行的条件。4.delete语句DELETEFROM table_nameWHERE some_column some_value;其中table_name是要删除数据的表名WHERE子句是用于指定要删除的行的条件。注意如果不指定条件相当于删除表中所有数据二、sql基本查询1.where子句WHERE子句是结构化查询语言SQL中用于筛选数据的关键字。通过WHERE子句您可以指定一个或多个条件来限制从数据库中检索的数据行。以下是WHERE子句的使用方式SELECT * FROM table_nameWHERE some_column some_value;1操作符WHERE子句可以使用以下比较运算符等于小于大于小于或等于大于或等于或!不等于还可以使用逻辑运算符AND、OR和NOT来组合多个条件。例如SELECT * FROM students WHERE sex ‘男’ and age 23;2模糊搜索-- 多字符匹配SELECT * FROM students WHERE clazz like xx%-- 单字符匹配SELECT * FROM students WHERE stu_name LIKE 张_;-- 多关键词匹配找姓名含“张”或“李”或“王”的学生SELECT * FROM students WHERE name RLIKE 张|李|王-- 范围匹配找姓名含“张”且后面跟1-2个字符的学生如“张三”“张三丰”SELECT * FROM students WHERE name RLIKE 张.{1,2};3in返回选项中的内容select * from students where clazz in (xx,xx,xx);4BETWEEN AND返回年龄在22到24的学生select * from students where age BETWEEN 22 AND 24;2.order by子句ORDER BY子句用于对查询结果按照一个或多个列进行排序。它接受一个或多个列名或表达式作为参数并可指定每个列的排序方式ASC:升序DESC:降序。语法如下SELECT column1, column2, ...FROM table_nameORDER BY column1 [ASC | DESC], column2 [ASC | DESC], ...3.group by子句GROUP BY子句用于将查询结果按照一个或多个列进行分组并对每个组进行聚合计算如COUNT、SUM、AVG等。语法如下SELECT column1, column2, ..., avg(column_name)FROM table_nameGROUP BY column1, column2, ...-- 嵌套whereselect * from(SELECT column1, column2, ..., aggregate_function(column_name) fFROM table_nameGROUP BY column1, column2, ...) as t1where t1.f xxx;4.having子句HAVING子句用于对分组后的结果进行过滤只返回符合条件的分组。它接受一个或多个聚合函数作为参数并可指定每个函数的过滤条件。语法如下SELECT column1, column2, ..., aggregate_function(column_name)FROM table_nameGROUP BY column1, column2, ...HAVING condition;5.limit子句LIMIT用于限制查询结果集的行数其中number是你想要返回的行数。其语法如下SELECT column1, column2, ...FROM table_nameLIMIT number;-- 多行查询 从number1起 limit number2个SELECT column1, column2, ...FROM table_nameLIMIT number1number2;三、数据连接1.外连接 内连接1内连接 join | inner joinSELECT s.*, sc.subject_id, su.subject_nameFROM (students sJOINscores scJOINsubjects suON s.stu_id sc.stu_id and sc.subject_id su.subject_id);2.外连接 left join / right join / full join# 在score末行添加一个假数据# 2000 1 100 期末SELECT *FROM(students as sRIGHT JOINscores as scON s.stu_id sc.stu_id);2.union 操作先拼接再去重删除重复行-- 合并“成绩≥90”和“成绩≤60”的学生信息UNION 自动去重SELECT s.stu_id, s.stu_name, sc.score, su.subject_nameFROM students sJOIN scores sc ON s.stu_id sc.stu_idJOIN subjects su ON sc.subject_id su.subject_idWHERE sc.score 90UNIONSELECT s.stu_id, s.stu_name, sc.score, su.subject_nameFROM students sJOIN scores sc ON s.stu_id sc.stu_idJOIN subjects su ON sc.subject_id su.subject_idWHERE sc.score 60ORDER BY score DESC;