Graphical SQL is too vivid

This paper introduces the design idea of relational database ： stay SQL in , It's all about .

There are many great design ideas and ideas in the computer field , for example ：

• stay Unix in , Everything is a document .
• In an object-oriented programming language , Everything is the object .

Relational databases also have their own design ideas ： stay SQL in , It's all about .

relational model

relational model （Relational model） from E.F.Codd Doctor Yu 1970 in , Based on the concept of relation in set theory ; Both the entity objects in the real world and the relations between them are represented by relations . The relationship we see in the database system is a two-dimensional table （Table）, By line （Row） And column （Column） form . therefore , It can also be said that a relational table is a collection of data rows .

The relational model consists of data structures 、 Relationship operation 、 Integrity constraints consist of three parts .

• The data structure in a relational model is a relational table , Including the basic table 、 Derived tables （ Query results ） And virtual tables （ View ）.

• Common relational operations include adding 、 Delete 、 Modification and query （CRUD）, What you use is SQL Language . The query operation is the most complex , Include options （Selection）、 Projection （Projection）、 Combine （Union）、 intersection （Intersection）、 Difference set （Exception） And Cartesian product （Cartesian product） etc. .

• Integrity constraints are used to maintain data integrity or meet business constraints , Including physical integrity （ Primary key constraint ）、 Referential integrity （ Foreign key constraints ） And user-defined integrity （ Non empty constraint 、 Unique constraint 、 Check constraints and defaults ）.

Our topic today is relational operating languages , That is to say SQL.

Face the set

SQL（ Structured query language ） Is the standard language for operating relational databases .SQL Very close to English , It's very simple to use . It is designed to take into account the needs of non-technical personnel at the beginning of its design , We usually just need to state the desired result （What）, And the process of data processing （How） Give it to the DBMS . So ,SQL Is the real programming language for people ！

Next, we will analyze the various operation statements of relations ; The purpose is to let people understand SQL Is a set oriented programming language , Its object of operation is a collection , The result of the operation is also a set .

In a relational database , Relationship 、 surface 、 Set three usually represent the same concept .

SELECT

Here is a simple query statement ：

SELECT employee_id, first_name, last_name, hire_date
   FROM employees;

Its function is to employees Query employee information in the table . obviously , We all know FROM And then there's a table （ Relationship 、 aggregate ）. More Than This , The result of the whole query statement is also a table . therefore , We can use the above query as a table ：

SELECT *
   FROM ( SELECT employee_id, first_name, last_name, hire_date
           FROM employees) t;

Queries in parentheses are called derived tables , We gave it an alias called t. Again , The entire query result is also a table ; This means that we can continue to nest , It's boring though .

Let's see one more PostgreSQL Example in ：

-- PostgreSQL
SELECT *
   FROM  upper( 'sql');
| upper |
| -------|
|   SQL |

upper() It's a capital conversion function . It appears again FROM clause , It means that its result is also a table , nothing but 1 That's ok 1 Special table of columns .

SELECT Clause is used to specify the fields to be queried , Can contain expressions 、 Function values, etc .SELECT In relational operations, it's called projection （Projection）, Look at the diagram below, it should be easier to understand .

except SELECT outside , There are also some commonly used SQL Clause .

WHERE Used to specify the conditions for data filtering , In relational operations it's called selection （Selection）, The schematic diagram is as follows ：

ORDER BY Used to sort the results of a query , The schematic diagram is as follows ：

All in all ,SQL Can complete all kinds of data operation , Such as filtering 、 grouping 、 Sort 、 Limit quantity, etc ; The objects of all these operations are relational tables , The result is also a relational table .

In these relational operations , There is a special one , Grouping is .

GROUP BY

grouping （ GROUP BY） Operations are different from other relational operations , Because it changes the structure of relationships . Take a look at the following example ：

SELECT department_id,  count(*), first_name
   FROM employees
  GROUP  BY department_id;

The purpose of this statement is to count the number of employees by Department , There's a syntax error, but there's a syntax error , Namely first_name Cannot appear in the list . The reason is that if you group by Department , Each department contains multiple employees ; Unable to determine which employee's name needs to be displayed , This is a logical mistake .

So ,GROUP BY Changed set elements （ data row ） Structure , Created a whole new relationship . The schematic diagram of grouping operation is as follows ：

For all that ,GROUP BY The result is still a collection .

UNION

SQL The most obvious manifestation of the set oriented feature is UNION（ Union operation ）、INTERSECT（ Intersection operation ） and EXCEPT/MINUS（ Subtraction operation ）.

The function of these set operators is to combine two sets into a set , So the following conditions need to be met ：

• The number and order of fields in both sets must be the same ;

• The types of the corresponding fields in the sets on both sides must match or be compatible with .

say concretely ,UNION and UNION ALL Used to compute the union of two sets , Returns the data that appears in the first query result or the second query result . The difference is UNION Eliminating duplicate data in the results ,UNION ALL Duplicate data is preserved . Here is UNION Schematic diagram of operation ：

INTERSECT The operator is used to return the common part of two sets , That is, the data that appears in both the first query result and the second query result , And it excludes duplicate data from the results .INTERSECT The schematic diagram of the operation is as follows ：

EXCEPT perhaps MINUS The operator is used to return the difference between two sets , It appears in the first query result , But not in the second query result , And it excludes duplicate data from the results .EXCEPT The schematic diagram of the operator is as follows ：

besides ,DISTINCT Operators are used to de duplicate data , That is, to exclude duplicate elements from the set .

SQL The concept of relation in mathematics comes from the set theory in mathematics , therefore UNION、INTERSECT and EXCEPT They come from Union in set theory （∪\cup∪）、 intersection （∩\cap∩） And subtraction （∖\setminus∖） operation . It should be noted that , Sets in set theory do not allow duplicate data , however SQL allow . therefore ,SQL Sets in are also called multisets （multiset）; Multiple sets and sets in set theory are disordered , however SQL Can pass ORDER BY Clause to sort the query results .

JOIN

stay SQL in , Not only are entity objects stored in relational tables , Relationships between objects are also stored in relational tables . therefore , When we want to get the relevant data , Need to use another operation ： Link query （JOIN）.

common SQL Join query type includes inner join 、 External connection 、 Cross connect, etc . among , External connection can be divided into left outer connection 、 Right outer connection and all outer connection .

Internal connection （Inner Join） Returns the data in two tables that satisfy the join condition , The principle of internal connection is shown in the figure below ：

The left outer join （Left Outer Join） Return all the data in the left table ; For the right table , Return the data that satisfies the connection condition ; If not, return null value . The principle of the left outer connection is shown in the figure below ：

Right connection （Right Outer Join） Return all the data in the right table ; For the left watch , Return the data that satisfies the connection condition , If not, return null value . The right outer connection and the left outer connection are interchangeable , The following two are equivalent ：

t1 RIGHT JOIN t2
t2 LEFT JOIN t1

Full outer join （Full Outer Join） It is equivalent to left outer join plus right outer join , All the data in the left and right tables are returned at the same time ; For data in two tables that do not meet the join condition, null value is returned . The principle of all external connection is shown in the figure below ：

Cross join is also called Cartesian product （Cartesian Product）. A cross join of two tables is equivalent to a pairwise combination of all rows in one table and all rows in another table , The number of results is multiplied by the number of rows in two tables . The principle of cross connection is shown in the figure below ：

There are other types of connections as well as semi connections （SEMI JOIN）、 Anti connect （ANTI JOIN）.

Set operations combine two sets into a larger or smaller set ; Join queries convert two sets into a larger or smaller collection , At the same time, you get a bigger element （ More columns ）. Most of the time, set operations can be implemented through join queries , for example ：

SELECT department_id
   FROM departments
  UNION
SELECT department_id
   FROM employees;

Equivalent to ：

SELECT  COALESCE(d.department_id, e.department_id)
   FROM departments d
   FULL  JOIN employees e  ON (e.department_id = d.department_id);

We've introduced many examples of queries , Let's take a look at other data operations .

DML

DML Represents a data manipulation language , That is to insert 、 Update and delete . Here's an example of inserting ：

CREATE  TABLE  test( id  int);

-- MySQL、SQL Server  etc.
INSERT  INTO  test( id)  VALUES ( 1),( 2),( 3);

-- Oracle
INSERT  INTO  test( id)
( SELECT  1  AS  id  FROM DUAL
UNION  ALL
SELECT  2  FROM DUAL
UNION  ALL
SELECT  3  FROM DUAL);

We passed a INSERT The statement inserts 3 Bar record , Or insert an inclusion 3 Relation table of records . because ,UNION ALL It returns a relational table .VALUES It also specifies a relational table , stay SQL Server and PostgreSQL The following statements are supported in ：

SELECT *
FROM (
   VALUES( 1),( 2),( 3)
)  test( id);

We've already said ,FROM And then there's a relationship table , So here VALUES Is the same . Because we often insert a single record , I don't realize that it's actually a table based operation .

Again ,UPDATE and DELETE Statements are also operations in relation tables ; It's just that we're used to talking about updating a row of data or deleting several records .