Basic database operations

SQL operations can be achieved:

operating	Language
Build a table	CREATE TABLE table (c1 int,c2 varchar(50),c3 varchar(50));
Delete table	DROP TABLE table;
Modify column	ALTER TABLE table ADD/DROP/ALERT column_name (datatype);
select	SELECT c1,c2 FROM table WHERE … ;
insert	INSERT INTO a VALUES(9,30,30);
Update	UPDATE table SET c1=value1,c2=value2,…WHERE… ;
delete	DELETE FROM table WHERE …;
connection	select * from [user] full JOIN phone ON [user].id=phone.id
WHERE
HAVING	Filter each group of data after grouping SELECT FROM WHERE GROUP BY HAVING;

Operation result processing	WHERE
AND	SELECT * FROM table WHERE c1=‘a’ AND c1=‘b’;
OR	SELECT * FROM table WHERE c1=‘a’ OR c1=‘b’;
Sort ORDER BY	Keyword is used to sort the result set according to one or more columns, in descending order ORDER BY column name DESC, column name DESC
Designated mode LIKE	Operators are used to search for the specified pattern in the column in the WHERE clause.
Return the number of rows COUNT(c1)	SELECT COUNT(c1) FROM table WHERE ;
Alias AS, AS can be omitted	Name AS alias, such as SELECT column name FROM table name AS alias;

SQL join is used to combine rows from two or more tables : INNER JOIN and JOIN (INNER JOIN and JOIN are the same
), LEFT JOIN (left full right empty), RIGHT JOIN, FULL JOIN

Insert picture description here

Processing view VIEW, index

trigger

Create TRIGGER [dbo].[Trigger_Insert]
   ON  a
   AFTER INSERT
AS 
BEGIN

	Declare @age int;  --@age为局部变量
    Select @age=Age  From inserted
	IF(@age<150)
		Begin
			Insert into a(id, name, age)
			Select ID, Name, Age From inserted
		END
	ELSE
		Begin
		    print('年龄应小于150')
			rollback transaction     --数据回滚
		END
    
END

SELECT name, AVG(age) AS age FROM a
GROUP BY name HAVING AVG(age) > 12

Insert picture description here

Online practice

other

Error handling
Insert picture description here
user is a keyword in sql, so an error will be reported. Generally, do not use keywords as table names. If keywords are used as table names, use [] to enclose

Writing norms

1. All lowercase SQL statements

2. Use single quotes when quoting characters

3. Add parentheses before and after the subquery

4. It is forbidden to use select * statements, and the specific fields of select must be pointed out. Avoid the use of'*' in the select clause. During the process of parsing, the database will convert'*' into all the column names in turn. This work is done by querying the data dictionary, which means that more time will be spent

5. It is forbidden to use insert into table values(?,?,?), you must point out the specific fields to be assigned, namely insert into tablea (col1, col2,…) values(?,?,…)

6. Avoid implicit type conversion. For example, the comparison or addition of numeric and int columns in the where clause, string conversion, etc.

7. Prohibit the use of views (views are basically related data between several tables. Very pressure for large data queries)

SQL database: basic database operation + writing specifications

Basic database operations

other

Writing norms

Guess you like