[data analysis] five common questions about learning SQL?

For small partners engaged in data work ,SQL Almost a necessary skill , Well written SQL It shows that you are a qualified ‘ Access migrant workers ’.

SQL How to go from rookie to expert , Just three steps ：

Familiar with basic addition, deletion, modification and query statements and functions , Include select、where、group by、having、order by、delete、insert、join、update etc. , You can do daily data retrieval or simple analysis （ This level has exceeded 90% Not IT colleagues ）;
Master and skillfully use high-level grammar , For example, a collection 、 Group aggregation 、 Subquery 、 Conditional logic 、 String function 、 Arithmetic functions 、 Date time function , And know MySQL、Oracle、SQL Server And other database syntax differences ;
Familiar with how to optimize SQL sentence , In order to achieve the highest query efficiency , Know the business 、 lock 、 Indexes 、 constraint 、 View 、 Metadata and other concepts , And learn to use hive sql、spark sql、pymysql Tools such as ;

It is better for data analysts to reach the third level , This can basically be a 90 Points of sqler.

If you put excel analogy sql, You have to play with the PivotTable 、 Various function formulas , even to the extent that VBA, To be able to work with maximum efficiency .

Study SQL There are laws to follow , You can start with the following five questions ：

1. Understand what is SQL？

SQL, Full name **「Structured Query Language」**, That is, structured query statements , Its main function is design , Create and manage relational databases , Tables in relational databases are similar excel A two-dimensional table of , It's made up of lines , Each column represents a field . let me put it another way ,SQL Is a programming language for communicating with relational databases .

Relational databases have many , such as MySQl、Oracle etc. , Each database uses its own SQL dialect , But they all share the same basic syntax .

SQL My responsibility is to manage the two-dimensional table , Replace... With a statement excel The graphical commands in .

For example, it is also filtering data , stay excel You need to add a filter to the column name in , Then select the desired field , This is a graphical command ;SQL Filter is realized through statements ;

select * from table where ....

SQL Grammar can be divided into three categories ：

The statement used to define the data structure is called **「SQL Scheme statement 」**, such as CREATE TABLE student...
Used to create 、 operation 、 A statement that retrieves data from a database is called **「SQL Data statements 」**, such as INSERTE INTO student...
Used to start 、 end 、 Or rolling back a transaction is called **「SQL Transaction statement 」**

Simple understanding , A schema statement is a shell for building data , In the example, a table is created student; Data statement is to process data , In the example, the table student Inserted data , Multiple statements solve multiple transactions SQL Disputes during execution .

If you're a database administrator , Then you need to master pattern statements at the same time 、 Data statements and transaction statements , But if you're just an analyst using data 、 The programmer , Then you only need to master the data statements .

SQL Not like a programming language , Using variables 、 Conditional logic 、 Cycle structure defines the process , To get the desired results .SQL Go straight up and down , Just define the necessary inputs and outputs , There is no control over the process .

2. How to learn SQL？

First you need to understand SQL The basis of grammar , Include SELECT,FROM,WHERE,GROUP BY,HAVING,ORDER BY、LIMIT etc. .

Beginners don't know much about abstract concepts , You can contrast excel Study SQL Basic grammar .

The core ：select * from tableName

This sentence is to select all fields from a table , amount to excel The choice in sheet

where amount to excel Screening ,group by amount to excel My PivotTable ,order amount to excel In ascending and descending order .

Then you have to learn some common functions ：

Finally, master advanced usage ：

3. What good books and tutorials are there ？

In the current market SQL There's a lot of relevant information , More fire like 《SQL Will know 》、《SQL Learning Guide 》 etc. .

There are also some good open source tutorials on the Internet , For example, rookie tutorial 、w3school etc. .

I suggest you finish a book or a set of tutorials , It can be done in a month .

4. What are they? SQL Actual combat website ？

Learn from practice SQL It's deadly , Maybe you've been studying for more than half a month , There is still no way to get a query demand , So practice is for SQL It's very important .

Here are two ways to practice SQL, First, build your own database , Then find a topic to practice , Second, in SQL Practice on the website

It's not difficult to build your own database , Beginners can install mysql, Then go to github Download exercises from .

SQL There are also many websites for practice , image SQLZOO、XUESQL、SQLBolt、leecode、 Niuke.com, etc .

SQL Tutorial/zh - SQLZOO