Easy introduction to SQL (1): addition, deletion, modification and simple query

The authors introduce

​ Whale song

E-commerce industry data analyst

Provide data consulting services for head brands

Keen to learn and share

“ DataMan creators Alliance ” member

Write in the pen SQL In series , I suddenly feel a little confused , Because in a sense SQL It's a stranger I'm familiar with . Familiar because I and SQL We met very early , Looking back on the whole process , We have experienced shallow acquaintance , Long forgotten in the Jianghu , Then there was a brief reunion , Then separation , Up to now, I once again fell under her pomegranate skirt .

Strangeness is because , Just like the process mentioned above , Many belong to me and SQL Because of the separation, my memories become light . This feeling is wonderful , But I'm glad that I finally understand SQL Position in my heart .

You may want to ask me , How to determine SQL Position in my heart ？ My answer is , Golden Circle thinking principle . Whether it's work or daily life , Rather than asking yourself why , We prefer to be guided by brain inertia , Take action first . And often many things come to the end , You will find that it was wrong from the beginning .

Used to be in school ,SQL It's just a course on the curriculum , Step into society ,SQL It's just a necessary tool for work , I have never thought deeply , Why should I choose SQL？SQL What does it mean to me .

• SQL He is a good comrade in arms for data analysis , No, SQL, Maybe we face the database , I can't wait to sigh ;

• SQL It's a high-level nonprocedural programming language , Master it , We can have one more mode of thinking ;

• SQL It is also the stepping stone and chip for our job search and salary increase , It may help you to take another step .

Now that you know SQL Importance to me , So in order not to feel strange anymore , The next period of time , I'll get to know it again SQL, And record these feelings in written form , Come on ！ Start the journey of revisiting ~

01  What is a database ？ What is? SQL？

database , As the name suggests, it is a warehouse for storing data , It's like an all inclusive storage box , Only its object is data . In the database , We are more exposed to relational databases , It includes multiple tables and the relationship between them . In the database , Relationship is the matching of data between two tables , For example, foreign keys .

SQL It is a tool for operating database , There are primitive and natural information in the database , And refining and processing these information depends on SQL.

02  How to install the database ？

In the choice of database , Usually preferred Mysql, It has the advantages of fast running speed and low cost . Specific installation , Please refer to the installation courses shared online .

Mysql And its good friends Navicat, They are the relationship between being managed and being managed , The former serves as a database system , The latter is a database management tool . Our daily basic operations on the database are Navicat In the middle of , It is powerful and easy to learn .

03  The creation of a table 、 Delete and update

1. Under the created database , You can right-click to select 【 new table 】：

Create table , The fields of the table will be set ：

See the following for the type selection ：

“ No null” If checked, it means that null value is not allowed , If it is not checked, it is allowed to be null .

A table can choose >=1 Column as primary key / Composite primary key , Ensure that each row of the table has identity uniqueness .

Null And empty string （“”） The difference is that empty characters have values and can be operated .

After the field is set , Click on 【 preservation 】, The skeleton of the table has been built , You need to fill in the corresponding data ：

The above data is similar to Excel The data entered in the table is the same , You need to input one by one manually .

2. Tables can be deleted ： Right click to select the table to delete ：

In practice ,【 Delete table 】 Be careful , Delete unrecoverable , Never owe your hand ！

3. and Excel equally , The data in the table has been changed , It needs to be updated , You can right-click the table , Update and save directly in the table .

04  Data insertion 、 Delete and update

In addition to adopting similar Excel In addition to data entry , You can also use query statements to insert 、 Delete and update ：

1. Insert into < Table name >（< Name 1>,< Name 2>,…）

Values（< value 1>,< value 2>,…）

• When inputting symbols, you can only use English , Remember that once there is an error, first check whether the symbol is in Chinese .

• A statement ends , In English semicolon （;） ending , Can't forget .

2. DELETE Statement to delete rows in a table ：DELETE FROM The name of the table WHERE Column name = value

3. Update Statement to modify the data in the table ：UPDATE The name of the table SET Column name = The new value WHERE Column name = Certain value

05  Simple query

The following covers 3 part ：

• School The database contains 4 Reports ：student、score、course and teacher, Report details and primary key ;

• SQL Knowledge point framework ： Basic statement 、 notes 、 Operators and fuzzy queries ;

• Screenshot of actual operation ： Correspond to the knowledge points one by one , Continuously improve from practice .

The following figure shows the current issue SQL The statement involves 4 Reports , Although the form is very small , In fact, the energy is huge , The real business scenario will involve more reports and massive data , The essence of the two is the same , The difference is the amount of data . If you can put 4 The actual operation of the report is smooth , So I'm not afraid to encounter a huge amount of data in the future ~

although SQL Easy to use , But as a database tool , We still have to practice makes perfect . Remember the story of the oil man ？ Weng said ：“ dedicated , But hand cooked. .” At work , Whether it's Excel、SQL still Python, We all need to practice while laying a solid foundation .

SQL Queries are generally targeted data retrieval , Rarely will you query the whole table , Because the amount of data presented is different from SQL The response speed is proportional , When the amount of data is very large, the response time will be longer , If you really need to see the whole table , It can be used limit Limit the number of rows ;

As In addition to setting the alias of the column , You can also set the alias of the table , Especially when it comes to multi table joins ;

Distinct Remember the placement of , In addition, the use of multiple columns may be ignored ;

Select You can also calculate the value directly ;

Not It's not commonly used , But you can also see in complex sentences , At this time, you should understand the meaning it represents ;

The priority of the operation cannot be mistaken , If it's a little windy , You can put it on the paper first where Write the clause well ;

Fuzzy query is used in many practical business scenarios , It is necessary to clarify what data to extract .

The old saying goes ： The fist does not leave the hand , practice singing all the time ,SQL Also practice often ！SQLZOO Is a good practice website , It's not important to know , It's good to do it , come on. ！