MySQL database - advanced query statement
- 1. Database query
- 2. Efficient query method
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- 1. Specify the field to query - SELECT
- 2. Perform deduplication query on the field - DISTINCT
- 3. Condition query - where
- 3. Increase query of logical relationship - and and or
- 4. Data record query of known value - IN
- 5. Data record query within the range - BETWEEN
- 6. Wildcard query
- 7. Keyword sorting query - ORDER BY
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- 3. Function query
- 4. Advanced query statement
- 5. Table join query
- Six, the use of view - view
- 7. Cascading - UNION
- 8. Intersection value - take the intersection of the results of two SQL statements
- Nine, case condition selection query statement
- 10. Regular expressions
1. Database query
A database is a tool used to store, update, and query data, and querying data is the core function of a database. The database is used to carry information, and the information is used for analysis and viewing. Therefore, it is necessary to master a more refined query method. This article will focus on advanced query statements for data.
2. Efficient query method
1. Specify the field to query - SELECT
Syntax: SELECT "字段" FROM "表名";
Example: SELECT Store_Name FROM fxk002;
2. Perform deduplication query on the field - DISTINCT
Syntax: SELECT DISTINCT "字段" FROM "表名";
Example: SELECT DISTINCT Store_Name FROM fxk003;
3. Condition query - where
Syntax: SELECT "字段" FROM "表名" WHERE "条件";
Example: SELECT Store_Name FROM fxk002 WHERE Sales > 1000;
3. Increase query of logical relationship - and and or
语法：SELECT "字段" FROM "表名" WHERE "条件1" {[AND|OR] "条件2"}+ ;
例：SELECT Store_Name FROM fxk003 WHERE Sales > 1000 OR (Sales < 500 AND Sales > 200);
4. Data record query of known value - IN
语法：SELECT "字段" FROM "表名" WHERE "字段" IN ('值1', '值2', ...);
例：SELECT * FROM fxk003 WHERE Store_Name IN (‘Los Angeles’, ‘Houston’);
5. Data record query within the range - BETWEEN
Syntax: SELECT "字段" FROM "表名" WHERE "字段" BETWEEN '值1' AND '值2';
Example: SELECT * FROM fxk003 WHERE Date BETWEEN '2020-12-06' AND '2020-12-10';
6. Wildcard query
Usually wildcards are used together with LIKE
| wildcard | illustrate |
|---|---|
| % | A percent sign means zero, one or more characters |
| _ | Underscores represent single characters |
LIKE——match a pattern to find the data records we want
Syntax: SELECT "字段" FROM "表名" WHERE "字段" LIKE {模式};
Example: SELECT * FROM fxk003 WHERE Store_Name like '%on%';
7. Keyword sorting query - ORDER BY
语法：SELECT "字段" FROM "表名" [WHERE "条件"] ORDER BY "字段" [ASC, DESC];
ASC 是按照升序进行排序的，是默认的排序方式。
DESC 是按降序方式进行排序。
例：SELECT Store_Name,Sales,Date FROM fxk003 ORDER BY Sales DESC;
3. Function query
1. Commonly used mathematical functions in the database
| math function | effect |
|---|---|
| abs(x) | returns the absolute value of x |
| rand() | Returns a random number between 0 and 1 |
| mod(x,y) | Returns the remainder after dividing x by y |
| power(x,y) | returns x to the power of y |
| round(x) | returns the integer closest to x |
| round(x,y) | Rounded value with x to y decimal places |
| sqrt(x) | returns the square root of x |
| truncate(x,y) | Returns the value of the number x truncated to y decimal places |
| ceil(x) | returns the smallest integer greater than or equal to x |
| floor(x) | returns the largest integer less than or equal to x |
| greatest(x1,x2…) | Returns the largest value in the collection, and can also return the largest value of multiple fields |
| least(x1,x2…) | Returns the smallest value in the collection, and can also return the smallest value of multiple fields |
2. Aggregation function:
| aggregate function | effect |
|---|---|
| avg() | Returns the mean of the specified column |
| count() | Returns the number of non-NULL values in the specified column |
| min() | Returns the minimum value of the specified column |
| max() | Returns the maximum value of the specified column |
| sum(x) | Returns the sum of all values in the specified column |
3. String function:
| string functions | effect |
|---|---|
| trim() | Returns the value with the specified format removed |
| concat(x,y) | Concatenate the provided parameters x and y into a string |
| substr(x,y) | Get the string starting from the yth position in the string x, which has the same effect as the substring() function |
| substr(x,y,z) | Gets a string of length z starting at position y in string x |
| length(x) | Returns the length of the string x |
| replace(x,y,z) | substitute the string z for the string y in the string x |
| upper(x) | convert all letters of the string x to uppercase |
| lower(x) | Convert all letters of the string x to lowercase |
| left(x,y) | Returns the first y characters of the string x |
| right(x,y) | Returns the last y characters of the string x |
| repeat(x,y) | repeat the string x y times |
| space(x) | returns x spaces |
| strcmp(x,y) | Compare x and y, the returned value can be -1,0,1 |
| reverse(x) | reverse the string x |
1. Remove character trim
1.去除字符 trim
语法格式：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。
2. 截取 substr
语法格式：SELECT substr(x,y) #截取x字符串 从第y个开始，截取到末尾
3.字段拼接 concat
语法格式： select concat(字段1,字段2) from 表名;
使用 || 符号
将info表中，name字段值和height字段值拼接在一起。
语法格式： select 字段1 || 字段2 from 表名;
将info表中，name字段值和height字段值拼接在一起，且中间加空格。
语法格式： select 字段1 || ' ' || 字段2 from 表名;
4.返回字符长度 length
语法格式： select length(字段1) from 表名;
5.替换 replace
select replace(字段1,'替换前字符段','替换后字符段') from 表名;
4. Advanced query statement
1. Grouping and summarizing - GROUP BY
Summarize and group the query results of the fields behind GROUP BY, which is usually used in combination with aggregate functions. There is
a principle in GROUP BY that all fields that appear after GROUP BY must appear after SELECT;
all fields that appear after SELECT and Fields that do not appear in aggregate functions must appear after GROUP BY
语法：SELECT "字段1", SUM("字段2") FROM "表名" GROUP BY "字段1";
例：SELECT Store_Name, SUM(Sales) FROM fxk003 GROUP BY Store_Name ORDER BY sales;
2.过滤——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。
语法：SELECT "字段1", SUM("字段2") FROM "表格名" GROUP BY "字段1" HAVING (函数条件);
例：SELECT Store_Name, SUM(Sales) FROM fxk003 GROUP BY Store_Name HAVING SUM(Sales) > 1500;
3.别名设置
----字段別名 表格別名
语法：SELECT "表格別名"."字段1" [AS] "字段別名" FROM "表格名" [AS] "表格別名";
例：SELECT A.Store_Name Store, SUM(A.Sales) “Total Sales” FROM fxk003 A GROUP BY A.Store_Name;
4.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 "字段1" FROM "表格1" WHERE "字段2" [比较运算符] 外查询
(SELECT "字段1" FROM "表格2" WHERE "条件"); 内查询
5.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 "字段1" FROM "表格1" WHERE EXISTS (SELECT * FROM "表格2" WHERE "条件");
例：SELECT SUM(Sales) FROM fxk003 WHERE EXISTS (SELECT * FROM fxk002 WHERE Region = ‘West’);
五、表连接查询
MYSQL数据库中常用的表连接有三种
1.inner join(内连接)
只返回两个表中联结字段相等的行
语法：select * from 表1 A inner join 表2 B on A.字段 = B.字段;
2.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
语法：select * from 表1 A left join 表2 B on A.字段 = B.字段;
3.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
语法：select * from 表1 A right join 表2 B on A.字段 = B.字段;
六、视图的运用—— 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，表格中有实际储存数据记录，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数据记录。
- 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SQL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。
语法格式：
创建视图表：CREATE VIEW "视图表名" AS "SELECT 语句";
删除视图表： DROP VIEW "视图表名";
七、联级——UNION
记录种类
UNION ：生成结果的数据记录值将没有重复，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 ：将生成结果的数据记录值都列出来，无论有无重复
语法：[SELECT 语句 1] UNION ALL [SELECT 语句 2];
八、交集值 ----取两个SQL语句结果的交集
1.联级视图求交集值
create view v_info as select distinct name from info union all select distinct name from info3;
select name,count(*) from v_info group by name;
select name from v_info group by name having count(*) >1;
2.内连接求交集值
select A.name from info A inner join info3 B on A.name=B.name;
select A.name from info A inner join info3 B using(name);
3.使用子查询的方式求交集值
4.取非交集值
- 联级方法中 count（*）<=1
- 左右内连接 将is not null 改为 is null
- 子查询 外连接查询 not in （内连接查询）
九、case 条件选择查询语句
SELECT CASE ("字段名")
WHEN "条件1" THEN "结果1"
WHEN "条件2" THEN "结果2"
[ELSE "结果N"]
END
FROM "表名";
# "条件"可以是一个数值或是公式。ELSE子句则并不是必须的
十、正则表达式
| 匹配模式 | 描述 | 实例 |
|---|---|---|
^ |
匹配文本的开始字符 | ‘^bd’ 匹配以 bd 开头的字符串 |
$ |
匹配文本的结束字符 | ‘qn$’ 匹配以 qn 结尾的字符串 |
. |
匹配任何单个字符 | ‘s.t’ 匹配任何 s 和 t 之间有一个字符的字符串 |
* |
matches zero or more of the characters preceding it | 'fo*t' matches t preceded by any o |
+ |
Matches the preceding character 1 or more times | 'hom+' matches a string starting with ho followed by at least one m |
字符串 |
matches the specified string | 'clo' matches strings containing clo |
p1丨p2 |
matches p1 or p2 | 'bg丨fg' matches bg or fg |
[...] |
Matches any character in the character set | '[abc]' matches a or b or c |
[^...] |
matches any character not in parentheses | '[^ab]' matches a string that does not contain a or b |
{n} |
Match the preceding string n times | 'g{2}' matches a string containing 2 g |
{n,m} |
Matches the preceding string at least n and at most m times | 'f{1,3}' matches f at least 1 time and at most 3 times |
grammar:SELECT "字段" FROM "表名" WHERE "字段" REGEXP {模式};
