The four basic core SQL statements (Basics)

Chapters:

Data query and filtering 1.
2. Data of polymerization
between 3 tables connected
by data 4, change, delete

learning target:

Master the common SQL statement, skilled call data in the database to solve the problem

The core SQL statements classified by function:

• Data retrieval: SELECT FROM
• Filter data: WHERE / HAVING
• Data packet: GROUP BY
• Summary function: aggregation function
• Data Sort: ORDER BY
• Data Select: LIMIT

SQL statements constitute the core and the order of execution

SELECT [DISTINCT]
	[列名],[聚合函数 AS <列名>]

FROM <表名>
[WHERE [<过滤条件>]
[GROUP BY <聚合类别列名>]
[HAVING <过滤条件>]
[ORDER BY<列名>(ASC|DESC) # 按照某一字段，升序或者降序排列]

[LIMIT <行数>]

# 执行顺序：

FROM > WHERE > GROUP BY > HAVING > SELECT(聚合函数) > ORDER BY > LIMIT

4 basic core SQL statements

Data query and filtering

data query:

• The basic query filter statements: the SELECT <column name> FROM <table name>

Data filtering

Filter statement based on a condition: the WHERE <Column name> value operator [AND / OR / NOT] column operator value

Query results handling

• Sorting Results: OEDER BY <column name> [ASC | DESC]
• Select specific results: LIMIT often used in conjunction with ORDER BY
• Query results deduplication: SELECT DISTINCT <column name> FROM <table name>

Data Aggregation

Syntax data aggregation

• SELECT <polymerization category> <aggregate functions ()> the FROM <table name>
• GROUP BY <polymerizable category>

Common aggregate functions:

• COUNT (l column names): Returns the number of rows in the specified column (not including NULL)
• COUNT (l *): Returns the number of rows in the specified column (including NULL)
• SUM (column names): Returns the value of the column total number (not including NULL)
• AVG (column names): Returns the value of the column average value (not including NULL)
• MIN / MAX (column names): returns the minimum / maximum value of the specified column (not including NULL)

Filter polymerization results:

• Use HAVING polymerization results and the filters filter

Sorting and selection polymerization results:

• Use ORDER BY and LIMIT

Inter-connect

• Equijoins: INNER JOIN - Returns the table on the left and the right key in the corresponding data table worth Airlines
• Left connector: LEFT JOIN - Returns the left and the right tables all rows of data corresponding to key value data table row
• Right connection: RIGHT JOIN - Returns the right and left rows form All the data table the row corresponding to the key value data

# 表间基本语法
SELECT<列名> FROM <左表名>

[INNER/ LEFT /RIGHT] JOIN <右表名>

ON <左表键值列名> = <右表键值列名>

Additions to delete data

Data table lists information:

# 列出数据表的信息：
DESCRIBE 表名
_________________

顾客号码	int(11)	NO
姓	varchar(50)	NO
名	varchar(50)	NO
城市	varchar(125)	YES
省份	varchar(125)	YES
电话	varchar(125)	YES

# 结果显示：1.数据表的字段（列）；2.各个字段的数据类型

Increased data line:
how to add new records to the table?

• Field and the list will record the number, the newly recorded data have the same type
• The new master key record (eg: customer number) value can not be the same as any of the primary key table present in

# 增加数据行

INSERT INTO 表名(字段名)VALUES(新记录的字段值
————————————————————————————————————————————————————————
# 示例
INSERT INTO `顾客信息`(`顾客号码`,`姓`,`名`,`城市`,`省份`,`电话`)VALUES
(92,'王','光明','长春市','吉林省',13812455432)

# 查询插入的数据表记录是否插入到数据表中

SELECT *
FROM `顾客信息`
WHERE `顾客号码` = 92

Modify the field value specifies the data row:

# 修改指定数据行的字段值
UPDATE 表名 SET 字段名 = 字段值[ WHERE 过滤条件]
————————————————————————————————————————————————————————————
# 第一步：找到需要修改的记录（行），目的避免误操作修改不该修改的记录

SELECT * FROM `顾客信息` WHERE `顾客号码` = 92

# 第二步：对满足条件的表记录进行修改
## 使用UPDATE SET 和 WHERE 子句实现
## 修改语句可用于修改一条/多条记录
## 新值的数据类型必须与表中定义好的数据类型保持一致；
————————————————————————————————————————————————————————————
# 修改指定数据行的字段值

UPDATE `顾客信息`
SET `电话` = 13700000000
WHERE `顾客号码` = 92

# 使用SELECT 和 WHERE 语句确定修改完成
SELECT *
FROM `顾客信息`
WHERE `顾客号码` = 92

Remove the specified data lines:

DELETE FROM 表名 [WHERE 过滤条件 ]
————————————————————————————————
# 删除指定数据行#
## 使用DELETE FROM 和 WHERE子句实现
## 删除语句可用于一条/多条记录；
## 删除后使用过滤语句获得的结果为空
————————————————————————————————
## 确认要删除的是目标记录,避免误操作
SELECT *
FROM `顾客信息`
WHERE `顾客号码` = 92

DELETE FROM `顾客信息`
WHERE `顾客号码` = 92

## 可以使用SELECT和 WHERE语句确定删除完成
SELECT *
FROM `顾客信息`
WHERE `顾客号码` = 92

[Note]: modify and delete statements must be included with WHERE filter criteria to ensure data security