SQL Complete guide for Interview

结构化查询语言或 SQL 是一种标准数据库语言，用于创建、维护、销毁、更新和检索关系数据库（如 MySQL、Oracle、SQL Server、PostgreSQL 等）中的数据。

实体关系模型（ER）

它是一个用于描述数据库中数据结构的概念框架。它旨在以更抽象的方式表示现实世界的实体及其之间的关系。这类似于编程语言的面向对象编程。

实体： 这些是现实世界中具有独特存在的对象或“事物”，例如客户、产品或订单。

关系： 这些定义了实体如何相互关联。例如，“客户”实体可能与“订单”实体有关系

命令：

创建数据库

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列出数据库

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使用数据库

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表的显示结构

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SQL 子语言

数据查询语言（DQL）：

用于对数据执行查询的语言。该命令用于从数据库中检索数据。

命令：

1) 选择：

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数据定义语言（DDL）：

用于定义数据库模式的语言。该命令用于创建、修改和删除数据库，但不用于数据。

命令

1）创建：

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2）掉落：
此命令完全删除表/数据库。

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3）截断：
此命令仅删除数据。

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4）改变：
该命令可以添加、删除或更新表的列。

添加

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修改

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掉落

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数据操作语言（DML）：

用于操作数据库中存在的数据的语言。

1）插入：
该命令用于仅插入新值。

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2）更新：

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3）删除：

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数据控制语言（DCL）：

GRANT：允许指定用户执行指定任务。
REVOKE：取消之前授予或拒绝的权限。

事务控制语言（TCL）：

它用于管理数据库中的事务。它管理 DML 命令所做的更改。

1）承诺
它用于将当前事务期间所做的所有更改保存到数据库中

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2）回滚
它用于撤消当前事务期间所做的所有更改

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3）保存点

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具有：

此命令用于根据聚合函数过滤结果。“我们不能在 WHERE 语句中使用聚合函数，因此我们可以在此命令中使用”
注意：当我们需要使用组成的列进行比较时可以使用此命令，而 WHERE 命令可以用于使用现有列进行比较

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在

当他们要求排除任何两个/更多特定项目时使用此命令

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清楚的：

此命令用于根据所选字段仅检索唯一数据。

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关联查询

它是一个子查询（嵌套在另一个查询中的查询），引用外部查询中的列

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正常化

标准化是一种数据库设计技术，用于以减少冗余并提高数据完整性的方式组织表。规范化的主要目标是将大表划分为更小、更易于管理的部分，同时保留数据之间的关系

第一范式（1NF）
列中的所有值都是原子的（不可分割的）。
每列仅包含一种类型的数据。

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1NF之后：

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第二范式（2NF）
它位于1NF。
所有非键属性在功能上完全依赖于主键（没有部分依赖）。

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2NF之后：

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第三范式（3NF）
它位于2NF。
所有属性在功能上仅依赖于主键（无传递依赖）。

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3NF之后：

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联盟：

该命令用于组合两个或多个 SELECT 语句的结果

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限制：

此命令用于限制从查询中检索的数据量。

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抵消：

该命令用于在返回结果之前跳过行数。

select Department, sum(Salary) as Salary
from employee
limit 2 offset 2;

Order By:

This command is used to sort the data based on the field in ascending or descending order.

Data:

create table employees (
    id int primary key,
    first_name varchar(50),
    last_name varchar(50),
    salary decimal(10, 2),
    department varchar(50)
);

insert into employees (first_name, last_name, salary, department)
values
    ('John', 'Doe', 50000.00, 'Sales'),
    ('Jane', 'Smith', 60000.00, 'Marketing'),
    ('Jim', 'Brown', 60000.00, 'Sales'),
    ('Alice', 'Johnson', 70000.00, 'Marketing');

select * from employees order by department;
select * from employees order by salary desc

Null

This command is used to test for empty values

select * from tablename
where colname IS NULL;

Group By

This command is used to arrange similar data into groups using a function.

select department, avg(salary) AS avg_salary
from employees
group by department;

Like:

This command is used to search a particular pattern in a column.

SELECT *
FROM employees
WHERE first_name LIKE 'a%';

SELECT *
FROM salesman
WHERE name BETWEEN 'A' AND 'L';

Wildcard:

Characters used with the LIKE operator to perform pattern matching in string searches.

% - Percent
_ - Underscore

How to print Wildcard characters?

SELECT 'It\'s a beautiful day'; 

SELECT * FROM table_name WHERE column_name LIKE '%50!%%' ESCAPE '!'; 

Case

The CASE statement in SQL is used to add conditional logic to queries. It allows you to return different values based on different conditions.

SELECT first_name, last_name, salary,
    CASE salary
        WHEN 50000 THEN 'Low'
        WHEN 60000 THEN 'Medium'
        WHEN 70000 THEN 'High'
        ELSE 'Unknown'
    END AS salary_category
FROM employees;

Display Text

1) Print something

Select "message";

select ' For', ord_date, ',there are', COUNT(ord_no)
group by colname;

2) Print numbers in each column

Select 1,2,3;

3) Print some calculation

Select 6x2-1;

4) Print wildcard characters

select colname1,'%',colname2
from tablename;

5) Connect two colnames

select first_name || ' ' || last_name AS colname
from employees

6) Use the nth field

select *
from orders
group by colname
order by 2 desc;

Constraints

1) Not Null:
This constraint is used to tell the field that it cannot have null value in a column.

create table employees(
    id int(6) not null
);

2) Unique:
This constraint is used to tell the field that it cannot have duplicate value. It can accept NULL values and multiple unique constraints are allowed per table.

create table employees (
    id int primary key,
    first_name varchar(50) unique
);

3) Primary Key:
This constraint is used to tell the field that uniquely identifies in the table. It cannot accept NULL values and it can have only one primary key per table.

create table employees (
    id int primary key
);

4) Foreign Key:
This constraint is used to refer the unique row of another table.

create table employees (
    id int primary key
    foreign key (id) references owner(id)
);

5) Check:
This constraint is used to check a particular condition for data to be stored.

create table employees (
    id int primary key,
    age int check (age >= 18)
);

6) Default:
This constraint is used to provide default value for a field.

create table employees (
    id int primary key,
    age int default 28
);

Aggregate functions

1)Count:

select count(*) as members from employees;

2)Sum:

select sum(salary) as total_amount
FROM employees;

3)Average:

select avg(salary) as average_amount
FROM employees;

4)Maximum:

select max(salary) as highest_amount
FROM employees;

5)Minimum:

select min(salary) as lowest_amount
FROM employees;

6)Round:

select round(123.4567, -2) as rounded_value;

Date Functions

1) datediff

select a.id from weather a join weather b on datediff(a.recordDate,b.recordDate)=1 where a.temperature > b.temperature;

2) date_add

select date_add("2017-06-15", interval 10 day);

SECOND
MINUTE
HOUR
DAY
WEEK
MONTH
QUARTER
YEAR    

3) date_sub

SELECT DATE_SUB("2017-06-15", INTERVAL 10 DAY);

Joins

Inner Join

This is used to combine two tables based on one common column.
It returns only the rows where there is a match between both tables.

Data

create table employees(
employee_id int(2) primary key,
first_name varchar(30),
last_name varchar(30),
department_id int(2)
);

create table department(
department_id int(2) primary key,
department_name varchar(30)
);

insert into employees values (1,"John","Dow",10);
insert into employees values (2,"Jane","Smith",20);
insert into employees values (3,"Jim","Brown",10);
insert into employees values (4,"Alice","Johnson",30);

insert into department values (10,"Sales");
insert into department values (20,"Marketing");
insert into department values (30,"IT");

select e.employee_id,e.first_name,e.last_name,d.department_name
from employees e
inner join department d
on e.department_id=d.department_id;

Left Join

This type of join returns all rows from the left table along with the matching rows from the right table. Note: If there are no matching rows in the right side, it return null.

select e.employee_id, e.first_name, e.last_name, d.department_name
from employees e
left join departments d
on e.department_id = d.department_id;

Right Join

This type of join returns all rows from the right table along with the matching rows from the left table. Note: If there are no matching rows in the left side, it returns null.

SELECT e.employee_id, e.first_name, e.last_name, d.department_name
FROM employees e
RIGHT JOIN departments d
ON e.department_id = d.department_id;

Self Join

This type of join is used to combine with itself especially for creation of new column of same data.

SELECT e.employee_id AS employee_id, 
       e.first_name AS employee_first_name, 
       e.last_name AS employee_last_name, 
       m.first_name AS manager_first_name, 
       m.last_name AS manager_last_name
FROM employees e
LEFT JOIN employees m ON e.manager_id = m.employee_id;

Full Join/ Full outer join

This type of join is used to combine the result of both left and right join.

SELECT e.employee_id, e.first_name, e.last_name, d.department_name
FROM employees e
FULL JOIN departments d
ON e.department_id = d.department_id;

Cross Join

This type of join is used to generate a Cartesian product of two tables.

SELECT e.name, d.department_name
FROM Employees e
CROSS JOIN Departments d;

Nested Query

A nested query, also known as a subquery, is a query within another SQL query. The nested query is executed first, and its result is used by the outer query.
Subqueries can be used in various parts of a SQL statement, including the SELECT clause, FROM clause, WHERE clause, and HAVING clause.

1) Nested Query in SELECT Clause:

SELECT e.first_name, e.last_name, 
       (SELECT d.department_name 
        FROM departments d 
        WHERE d.id = e.department_id) AS department_name
FROM employees e;

2) Nested Query in WHERE Clause:

SELECT first_name, last_name, salary
FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);

SELECT pro_name, pro_price
FROM item_mast
WHERE pro_price = (SELECT MIN(pro_price) FROM item_mast); 

3) Nested Query in FROM Clause:

SELECT department_id, AVG(salary) AS avg_salary
FROM employees
GROUP BY department_id;

4) Nested Query with EXISTS:

SELECT customer_name
FROM customers c
WHERE EXISTS (
    SELECT 1
    FROM orders o
    WHERE o.customer_id = c.customer_id
);

Exists

This command is used to test the existence of a particular record. Note: When using EXISTS query, actual data returned by subquery does not matter.

SELECT customer_name
FROM customers c
WHERE EXISTS (
    SELECT 1
    FROM orders o
    WHERE o.customer_id = c.customer_id
);

SELECT customer_name
FROM customers c
WHERE NOT EXISTS (
    SELECT 1
    FROM orders o
    WHERE o.customer_id = c.customer_id
);

COALESCE

The COALESCE function in SQL is used to return the first non-null expression among its arguments. It is particularly useful for handling NULL values and providing default values when dealing with potentially missing or undefined data.

CREATE TABLE employees (
    first_name VARCHAR(50),
    middle_name VARCHAR(50),
    last_name VARCHAR(50)
);

INSERT INTO employees (first_name, middle_name, last_name) VALUES
('John', NULL, 'Doe'),
('Jane', 'Marie', 'Smith'),
('Emily', NULL, 'Johnson');

SELECT 
    first_name,
    COALESCE(middle_name, 'No Middle Name') AS middle_name,
    last_name
FROM 
    employees;

PL/SQL(Procedural Language/Structured Query Language)

It is Oracle's procedural extension to SQL. If multiple SELECT statements are issued, the network traffic increases significantly very fast. For example, four SELECT statements cause eight network trips. If these statements are part of the PL/SQL block, they are sent to the server as a single unit.

Blocks

They are the fundamental units of execution and organization.

1) Named block
Named blocks are used when creating subroutines. These subroutines are procedures, functions, and packages. The subroutines can be stored in the database and referenced by their names later on.

Ex.

CREATE OR REPLACE PROCEDURE procedure_name (param1 IN datatype, param2 OUT datatype) AS
BEGIN
   -- Executable statements
END procedure_name;

2) Anonymous
They are blocks do not have names. As a result, they cannot be stored in the database and referenced later.

DECLARE
   -- Declarations (optional)
BEGIN
   -- Executable statements
EXCEPTION
   -- Exception handling (optional)
END;

Declaration
It contains identifiers such as variables, constants, cursors etc
Ex.

declare
v_first_name varchar2(35) ;
v_last_name varchar2(35) ;
v_counter number := 0 ;
v_lname students.lname%TYPE; // takes field datatype from column

Rowtype

DECLARE
v_student students%rowtype;

BEGIN

select * into v_student
from students
where sid='123456';
DBMS_OUTPUT.PUT_LINE(v_student.lname);
DBMS_OUTPUT.PUT_LINE(v_student.major);
DBMS_OUTPUT.PUT_LINE(v_student.gpa);

END;

Execution
It contains executable statements that allow you to manipulate the variables.

declare
v_regno number;
v_variable number:=0;
begin
select regno into v_regno from student where regno=1;
dbms_output.put_line(v_regno || ' '|| v_variable);
end

Input of text

DECLARE
v_inv_value number(8,2);
v_price number(8,2);
v_quantity number(8,0) := 400;

BEGIN
v_price := :p_price;
v_inv_value := v_price * v_quantity;
dbms_output.put_line(v_inv_value);
END;

If-else loop

      IF rating > 7 THEN 
         v_message := 'You are great'; 
      ELSIF rating >= 5 THEN 
         v_message := 'Not bad'; 
      ELSE 
          v_message := 'Pretty bad'; 
      END IF;  

Loops

Simple Loop

declare
begin
    for i in 1..5 loop
        dbms_output.put_line('Value of i: ' || i);
    end loop;
end;

While Loop

declare
    counter number := 1;
begin
    while counter <= 5 LOOP
        dbms_output.put_line('Value of counter: ' || counter);
        counter := counter + 1;
    end loop;
end;

Loop with Exit

declare
    counter number := 1;
begin
    loop
        exit when counter > 5; 
        dbms_output.put_line('Value of counter: ' || counter);
        counter := counter + 1; 
    end loop;
end;

Procedure

A series of statements accepting and/or returning
zero variables.

--creating a procedure
create or replace procedure proc (var in number) as
begin
dbms_output.put_line(var);
end

--calling of procedure
begin
proc(3);
end

Function

A series of statements accepting zero or more variables that returns one value.

create or replace function func(var in number)
return number
is res number;
begin 
select regno into res from student where regno=var;
return res;
end

--function calling
declare
var number;
begin
var :=func(1);
dbms_output.put_line(var);
end

All types of I/O

p_name IN VARCHAR2
p_lname OUT VARCHAR2
p_salary IN OUT NUMBER

Triggers

DML (Data Manipulation Language) triggers are fired in response to INSERT, UPDATE, or DELETE operations on a table or view.

BEFORE Triggers:
Execute before the DML operation is performed.
AFTER Triggers:
Execute after the DML operation is performed.
INSTEAD OF Triggers:
Execute in place of the DML operation, typically used for views.

Note: :new represents the cid of the new row in the orders table that was just inserted.

create or replace trigger t_name
after update on student
for each row
begin
dbms_output.put_line(:NEW.regno);
end

--after updation
update student
set name='name'
where regno=1;

Window function

SELECT
    id,name,gender,
    ROW_NUMBER() OVER(
         PARTITION BY name
         order by gender
    ) AS row_number
FROM student;

SELECT
    employee_id,
    department_id,
    salary,
    RANK() OVER(
        PARTITION BY department_id
        ORDER BY salary DESC
    ) AS salary_rank
FROM employees;

ACID Properties:

Atomicity:
All operations within a transaction are treated as a single unit.
Ex. Consider a bank transfer where money is being transferred from one account to another. Atomicity ensures that if the debit from one account succeeds, the credit to the other account will also succeed. If either operation fails, the entire transaction is rolled back to maintain consistency.

Consistency:
Consistency ensures that the database remains in a consistent state before and after the transaction.
Ex. If a transfer transaction reduces the balance of one account, it should also increase the balance of the receiving account. This maintains the overall balance of the system.

Isolation:
Isolation ensures that the concurrent execution of transactions results in a system state that would be obtained if transactions were executed serially, i.e., one after the other.
Ex. Consider two transactions T1 and T2. If T1 transfers money from account A to account B, and T2 checks the balance of account A, isolation ensures that T2 will either see the balance of account A before the transfer (if T1 has not committed yet) or after the transfer (if T1 has committed), but not an intermediate state.

Durability:
Durability guarantees that once a transaction is committed, its effects are permanent and survive system failures. Even if the system crashes or restarts, the changes made by the transaction are not lost.

Data Types

1) Numeric Datatype
int
decimal(p,q) - p is size, q is precision

2) String Datatype
char(value) - max(8000) && immutable
varchar(value) - max(8000)
text - largest size

3) Date Datatype
date
time
datetime

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