函数
数学函数
abs(x) #返回x的绝对值 rand() #返回0到1的随机数 mod(x,y) #返回x除以y以后的余数 power(x,y) #返回x的y次方 round(x) #返回离x最近的整数 round(x,y) #保留x的y位小数四舍五入后的值 sqrt(x) #返回x的平方根 truncate(x,y) #返回数字x截断为y位小数的值 ceil(×) #返回大于或等于x的最小整数 floor(x) #返回小于或等于x的最大整数 greatest(x1,x2...) #返回集合中最大的值 least(x1,x2...) #返回集合中最小的值 SELECT abs(-1),rand(),mod(5,3),power(2,3),round(1.89); SELECT round(1.8937,3),truncate(1.235,2),ceil(5.2),floor(2.1),least(1.89,3,6.1,2.1);
聚合函数
聚合函数: avg() #返回指定列的平均值 count() #返回指定列中非 NULL值的个数 min() #返回指定列的最小值 max() #返回指定列的最大值 sum(x) #返回指定列的所有值之和 SELECT avg(Sales) FROM Store_Info; SELECT count(store_Name) FROM Store_Info; SELECT count(DISTINCT store_Name) FROM Store_Info; SELECT max(Sales) FROM Store_Info; SELECT min(sales) FROM Store_Info; SELECT sum(sales) FROM Store_Info; SELECT count(DISTINCT store_Name) FROM Store_Info; SELECT count(*) FROM Store_Info; #count(*)包括了所有的列的行数,在统计结果的时候,不会忽略列值为NULL #count(列名)只包括列名那一列的行数,在统计结果的时候,会忽略列值为NULL的行
字符串函数
字符串函数: trim() #返回去除指定格式的值 concat(x,y) #将提供的参数x和y拼接成一个字符串 substr(x,y) #获取从字符串x中的第y个位置开始的字符串,跟substring()函数作用相同 substr(x,y,z) #获取从字符串x中的第y个位置开始长度为z的字符串 length(x) #返回字符串x的长度 replace(x,y,z) #将字符串z替代字符串x中的字符串y upper(x) #将字符串x的所有字母变成大写字母 lower(x) #将字符串x的所有字母变成小写字母 left(x,y) #返回字符串x的前y个字符 right(x,y) #返回字符串x的后y个字符 repeat(x,y) #将字符串x重复y次 space(x) #返回x个空格 strcmp (x,y) #比较x和y,返回的值可以为-1,0,1 reverse(x) #将字符串x反转 SELECT concat(Region,Store_Name) FROM location WHERE Store_Name = 'Boston'; #如sql_mode开启开启了PIPES_AS_CONCAT,"||"视为字符串的连接操作符而非或运算符,和字符串的拼接函数concat相类似,这和Oracle数据库使用方法一样的 SELECT Region || ' ' || Store_Name FROM location WHERE Store_Name = 'Boston'; SELECT substr(Store_Name,3) FROM location WHERE Store_Name = 'Los Angeles'; SELECT substr(Store_Name,2,4) FROM location WHERE Store_Name = 'New York'; SELECT TRIM ([ [位置] [要移除的字符串] FROM ] 字符串); #[位置]:的值可以为 LEADING(起头),TRAILING(结尾),BOTH(起头及结尾)。 #[要移除的字符串]:从字串的起头、结尾,或起头及结尾移除的字符串。缺省时为空格 SELECT TRIM(LEADING 'Ne' FROM 'New York'); SELECT Region,length(Store_Name) FROM location; SELECT REPLACE(Region,'ast','astern')FROM location;
---- GROUP BY ---- 对GROUP BY后面的栏位的查询结果进行汇总分组,通常是结合聚合函数一起使用的 GROUP BY有一个原则,就是SELECT后面的所有列中,没有使用聚合函数的列,必须出现在GROUP BY后面。 语法:SELECT "栏位1",SUM("栏位2") FROM "表名" GROUP BY "栏位1"; SELECT Store_Name,SUM(Sales) FROM Store_Info GROUP BY Store_Name ORDER BY sales desc; ---- HAVING ---- 用来过滤由GROUP BY语句返回的记录集,通常与GROUP BY语句联合使用 HAVING语句的存在弥补了wHERE关键字不能与聚合函数联合使用的不足。如果被SELECcT的只有函数栏,那就不需要GROUP BY子句。 语法:SELECT "栏位1",SUM("栏位2") FROM "表格名" GROUP BY "栏位1" HAVING (函数条件); SELECT Store_Name,SUM(Sales) FROM Store_Info GROUP BY Store_Name HAVING SUM (Sales) > 1500; ---- 别名 ---- 栏位别名表格别名 语法:SELECT "表格别名"."栏位1” [AS] "栏位别名" FROM "表格名" [AS] "表格别名" SELECT A.Store_Name Store,SUM(A.Sales) "Total Sales" FROM Store_Info A GROUP BY A.Store_Name; ---- 子查询 ---- 连接表格,在WHERE子句或 HAVING子句中插入另一个 SQL语句 语法: SELECT "栏位1" FROM "表格1" WHERE "栏位2" [比较运算符] #外查询 (SELECT "栏位1" FROM "表格2" WHERE "条件"); #内查询 可以是符号的运算符,例如 =、>、<、>=、<= ;也可以是文字的运算符,例如LIKE、IN、BETWEEN SELECT SUM(Sales) FROM Store_Info WHERE Store_Name IN (SELECT Store_Name FROM location WHERE Region = 'west'); SELECT SUM(A.Sales) FROM Store_Info A WHERE A.Store_Name IN (SELECT Store_Name FROM location B WHERE B.Store_Name = A.Store_Name);
EXISTS
---- EXISTS ---- 用来测试内查询有没有产生任何结果,类似布尔值是否为真 #如果有的话,系统就会执行外查询中的SQL语句。若是没有的话,那整个SQL语句就不会产生任何结果。 语法: SELECT "栏位1" FROM "表格1" WHERE EXISTS (SELECT * FROM "表格2" WHERE "条件"); SELECT SUM(Sales) FROM Store_Info WHERE EXISTS (SELECT * FROM location WHERE Region = 'West');
- 连接查询
- location 表格
UPDATE Store_Info SET store_name='washington’WHERE sales=300;
- 1
- Store_Info表格
inner join(内连接):只返回两个表中联结字段相等的行 left join(左连接):返回包括左表中的所有记录和右表中联结字段相等的记录 right join(右连接):返回包括右表中的所有记录和左表中联结字段相等的记录 SELECT * FROM location A INNER JOIN Store_Info B on A.Store_Name = B.store_Name ; SELECT * FROM location A RIGHT JOIN Store_Info B on A.Store_Name = B.Store_Name ; SELECT * FROM location A,store_Info B WHERE A.Store_Name = B.Store_Name; SELECT A.Region REGION,SUM(B.Sales) SALES FROM location A,Store_Info B WHERE A.Store_Name = B.Store_Name GROUP BY REGION;
CREATE VIEW
---- CREATE VIEW ---- 视图,可以被当作是虚拟表或存储查询。 ·视图跟表格的不同是,表格中有实际储存资料,而视图是建立在表格之上的一个架构,它本身并不实际储存资料。 ·临时表在用户退出或同数据库的连接断开后就自动消失了,而视图不会消失。 ·视图不含有数据,只存储它的定义,它的用途一般可以简化复杂的查询。比如你要对几个表进行连接查询,而且还要进行统计排序等操作,写SQT语句会很麻烦的,用视图将几个表联结起来,然后对这个视图进行查询操作,就和对一个表查询一样,很方便。 语法:CREATE VIEW "视图表名” AS "SELECT语句"; CREATE VIEW V_REGION_SALES AS SELECT A.Region REGION, SUM(B.Sales) SALES FROM location A INNER JOIN Store_Info B ON A.Store_Name = B.Store_Name GROUP BY REGION; SELECT * FROM V_REGION_SALES; DROP VIEW V_REGION_SALES;
UNION
---- UNION ---- 联集,将两个SQL语句的结果合并起来,两个SQI语句所产生的栏位需要是同样的资料种类 UNION:生成结果的资料值将没有重复,且按照字段的顺序进行排序 语法:[SELECT 语句 1] UNION [SELECT 语句2]; UNION ALL:将生成结果的资料值都列出来,无论有无重复 语法:[SELECT 语句 1] UNION ALL [SELECT 语句 2]; SELECT Store_Name FROM location UNION SELECT Store_Name FROM Store_Info; SELECT Store_Name FROM location UNION ALL SELECT Store_Name FROM Store_Info;
交集值
----- 交集值 ---- 取两个SQL语句结果的交集 SELECT A.Store_Name FROM location A INNER JOIN Store_Info B ON A.Store_Name = B.Store_Name; SELECT A.Store_Name FROM location A INNER JOIN Store_Info B USING(Store_Name); #两表基中的一个表没有指定的行,而另一个表这个行有重复,并且确实有交集的时候用 SELECT A.Store_Name FROM (SELECT Store_Name FROM location UNION ALL SELECT Store_Name FROM store_Info) A GROUP BY A.Store_Name HAVING COUNT(*) > 1; #取两个sQL语句结果的交集,且没有重复 SELECT A.Store_Name FRONM (SELECT B.Store_Name FROM location B INNER JOIN Store_Info C ON B.Store_Name = C.store_Name) A GROUP BY A.Store_Name; SELECT DISTINCT A.Store_Name FROM location A INNER JOIN Store_Info B USING(Store_Name); SELECT DISTIMCT Store_Name FROM location WHERE (Store_Name) IN (SELECT Store_Name FROM Store_Info); SELECT DISTINCT A.Store_Name FROM location A LEFT JOIN Store_Info B USING(Store_Name) WHERE B.Store_Name IS NOT NULL;
无交集值
---- 无交集值 ---- 显示第一个sQL语句的结果,且与第二个SQL语句没有交集的结果,且没有重复 SELECT DISTINCT Store_Name FROM location WHERE (Store_Name) NOT IN (SELECT Store_Name FROM Store_Info); SELECT DISTINCT A.Store_Name FROM location A LEFT JOIN Store_Info B USING(Store_Name) WHERE B.Store_Name IS NULL;