SQL Tutorial for Beginners with MySQL

SQL Tutorial for Beginners with MySQL

SQL Tutorial for Beginners with MySQL

1- Introduction

This document guides for beginners to learn SQL. The document is based on:

  • MySQL >= 5.6

Its content includes

  • SQL language
  • SQL syntax (Basic syntax, standard for every DB)
  • Specific SQL syntax of MySQL.

2- Database to learn SQL (LearningSQL)

This Post use 
LearningSQL database (
MySQL version). You need to create that database for the convenience of your studying (It takes you less than 5 minutes to create it).

Script create database. The structure of this database can be seen at:

3- Visualization tools to work with MySQL

In this instruction, I use 
MySQL Workbench which is visual tool to work. You can see more instructions for installing 
MySQL Community (After the install is finished, 
MySQL Workbench is also included in) at:

4- Query

4.1- SQL Select

Select statement is the most basic statement in SQL. It is used to query data in tables.
  • PRODUCT_TYPE: A data table of types of products (Bank’s service).
This is Select statement, query data in the table
PRODUCT_TYPE
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-- Query entire column of Product_Type table.
Select * From Product_Type;
-- The above statement and the following statement are equivalent.
-- Pty is alias (a alias of Product_Type table).
Select Pty.* From Product_Type Pty;
-- Query some columns.
Select Pty.Product_Type_Cd
     ,Pty.Name
From   Product_Type Pty;
Query results:
Query
EMPLOYEE table:
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-- Query 4 columns on the Employee table
-- Using emp as alias for Employee
Select Emp.Emp_Id
     ,Emp.First_Name
     ,Emp.Last_Name
     ,Emp.Dept_Id
From   Employee Emp;
Query results:

Create Alias for column:

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-- Using the concat (string1, string2) to  concatenate two strings together.
-- We have a new column, use 'as' to define column name for this column.
Select Emp.Emp_Id
   ,Emp.First_Name
   ,Emp.Last_Name
   ,Emp.Dept_Id
   ,Concat('EMP'
          ,Emp.Emp_Id) As Emp_No2  -- New column
From   Employee Emp;
-- Maybe not need 'as' in the definition Alias column.
Select Emp.Emp_Id
   ,Emp.First_Name
   ,Emp.Last_Name
   ,Emp.Dept_Id
   ,Concat('EMP'
          ,Emp.Emp_Id)  Emp_No2  -- New column
From   Employee Emp;
Query results:

4.2- SQL Distinct

Distinct statement is used along with Select in order to select data and skip identical data. Syntax is:
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Select distinct ,   ....
View Example:
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-- Query Product table.
Select Pro.Product_Cd
     ,Pro.Name
     ,Pro.Product_Type_Cd
From   Product Pro;
-- Query Product_Type in Product table.
Select Pro.Product_Type_Cd from Product Pro;
-- Distinct need to use to remove the duplicates.
Select Distinct Pro.Product_Type_Cd from Product Pro;
Results of running the above commands:

4.3- SQL Where

Where is a statement aimed at limiting the scope of finding. For example, you want to look for service products that are
“Individual an Business Loans”

  • Product_Type_Cd = ‘LOAN’.

You need to query in
PRODUCT table, where
Product_Type_Cd= ‘LOAN’.

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-- Querying the Product table to find the type of products:
-- "Individual and Business Loans".
-- Corresponding column: Product_Type_Cd = 'LOAN'.
Select * From Product Pro Where Pro.Product_Type_Cd = 'LOAN';
Query results:
Another example used where and conditions

4.4- SQL And Or

And and
Or are operators used in
where:
For example, you want to make a list looking for employees whose first
name starts with ‘S’ character and work in Operation department.
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-- Query the employee whose first name starts with S.
Select Emp.Emp_Id
     ,Emp.First_Name
     ,Emp.Last_Name
     ,Emp.Dept_Id
From   Employee Emp
Where  Emp.First_Name Like 'S%';
-- Query the employee whose first name starts with S.
-- and work in Operation department.  Dept_Id  = 1.
Select Emp.Emp_Id
     ,Emp.First_Name
     ,Emp.Last_Name
     ,Emp.Dept_Id
From   Employee Emp
Where  Emp.First_Name Like 'S%'
And    Emp.Dept_Id = 1;
Results of running the queries:
Results of running the queries:
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-- Find the employees whose first name starts with S or P.
-- and work in Operation department.  Dept_Id  = 1.
Select Emp.Emp_Id
     ,Emp.First_Name
     ,Emp.Last_Name
     ,Emp.Dept_Id
From   Employee Emp
Where  (Emp.First_Name Like 'S%' Or Emp.First_Name Like 'P%')
And    Emp.Dept_Id = 1;
Query Results:

4.5- SQL IN

The
IN operator allows testing a term of a condition by comparing it for equality with a list of fixed values.
IN works with values of all data types
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-- This command searches the employee named
-- Susan or  Paula or Helen.
Select Emp.Emp_Id
     ,Emp.First_Name
     ,Emp.Last_Name
     ,Emp.Dept_Id
From   Employee Emp
Where  Emp.First_Name In ('Susan'
                        ,'Paula'
                        ,'Helen');
Query Results:

4.6- SQL Between

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-- Find employees, with Emp_Id between 5 and 10.
Select Emp.Emp_Id
    ,Emp.First_Name
    ,Emp.Last_Name
    ,Emp.Start_Date
From   Employee Emp
Where  (Emp.Emp_Id Between 5 And 10);
-- The statement above is equivalent to:
Select Emp.Emp_Id
     ,Emp.First_Name
     ,Emp.Last_Name
     ,Emp.Start_Date
From   Employee Emp
Where  Emp.Emp_Id >= 5
And    Emp.Emp_Id <= 10;
Query Results:
BETWEEN statement is also used in time limit. For example, you look for employees who begin working in the period of time from:
  • 03-05-2002 ==> 09-08-2002  (dd-MM-yyyy)
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-- This statement helps find employees who have begun working for a period of time
-- specify it in where statement.
-- For example,  03-05-2002 ==> 09-08-2002  (Format: dd-MM-yyyy)
Select Emp.Emp_Id
     ,Emp.First_Name
     ,Emp.Last_Name
     ,Emp.Start_Date
     , -- DATE_FORMAT is a function of MySQL .
       -- It may not exist in other DB.
       -- DATE_FORMAT convert Date type to text
       -- According to a certain format here is DD-MM-YYYY
      Date_Format(Emp.Start_Date
             ,'%d-%m-%Y') Start_Date_Vn
From   Employee Emp
Where
-- Str_To_Date is function of MySQL (It may not exist in other DB)
-- Str_To_Date convert Text to Date
-- According to a certain format here is DD-MM-YYYY
( --
 Emp.Start_Date Between Str_To_Date('03-05-2002'
                               ,'%d-%m-%Y') And
 Str_To_Date('09-08-2002'
        ,'%d-%m-%Y') --
);
Query Results:

4.7- SQL Wildcard

There are 2 special characters in SQL:

  1. % character
  2. _ character

Meaning:

  • % describes 0, 1 or more characters
  • _ describes exactly one character.

Two characters are usually used in the
LIKE condition.

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-- Find Customers whose FED_ID is formatted:
-- The initial part is random, following by -, then two any characters, then -, and the last part is any.
-- Use two dashes (_) for illustrating two characters.
-- (Each dash (_) is a unique character).
Select Cus.Cust_Id
     ,Cus.Fed_Id
     ,Cus.Address
From   Customer Cus
where cus.fed_id like '%-__-%';
Query Results:

4.8- SQL Like

This statement we have become familiar in some of the examples above.

4.9- SQL Order By

ORDER BY clause: Arranging the result of a query in order.
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-- Syntax:
SELECT "column_name"
FROM "table_name"
[WHERE "condition"]
ORDER BY "column_name1" [ASC, DESC], "column_name2" [ASC, DESC];
-- Note:
-- ASC: ascending (default)
-- DESC: descending order..
Example:
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-- Arrange Product_Type_Cd in ascending order
-- Next, arrange Name in ascending order, too.
Select Pro.Product_Cd
    ,Pro.Product_Type_Cd
    ,Pro.Name
From   Product Pro
Order  By Pro.Product_Type_Cd Asc
       ,Pro.Name            Asc;
      
      
-- In Order BY, ASC is defaulted.
-- Thus, it is unnecessary to write ASC.
Select Pro.Product_Cd
    ,Pro.Product_Type_Cd
    ,Pro.Name
From   Product Pro
Order  By Pro.Product_Type_Cd
       ,Pro.Name;
-- Arrange Product_Type_Cd in descending order
-- Next, arrange Name in ascending order
Select Pro.Product_Cd
    ,Pro.Product_Type_Cd
    ,Pro.Name
From   Product Pro
Order  By Pro.Product_Type_Cd Desc
       ,Pro.Name            Asc;
Query Results:
Order By always stands behind
where.
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-- Find the employees whose first name starts with S.
-- Sort descending by start date of work.
Select Emp.Emp_Id
     ,Emp.First_Name
     ,Emp.Last_Name
     ,Emp.Start_Date
From   Employee Emp
Where  Emp.First_Name Like 'S%'
Order  By Emp.Start_Date Desc;
Query Results:
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-- Apply the order of column to Order by clause.
-- First_Name is the second column in Select clause.
-- Be able to use Order by 2 in lieu of Order by First_Name.
Select Emp.Emp_Id
    ,Emp.First_Name
    ,Emp.Last_Name
    ,Emp.Start_Date
From   Employee Emp
Order  By 2 Desc;

4.10- SQL Group By

First, we need to understand what
Aggregate Functions are:

  • Sum: Function returns the sum of all the values
  • Avg: Function return average of values
  • Count: Function Returns the number of items.
  • Min: Function finds the minimum value
  • Max: Function finds the maximum value

Those are some common
aggregate functions. They can participate in
Group by statement.

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-- Query Account table.
Select Acc.Account_Id
    ,Acc.Product_Cd
    ,Acc.Avail_Balance
    ,Acc.Pending_Balance
From   Account Acc;
The question is that you want to see the total amount of money in
account in proportion to each type of different services (Product_Cd).
It means you need to group on Product_Cd.
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Select Acc.Product_Cd
     ,Count(Acc.Product_Cd) As Count_Acc
     ,Sum(Acc.Avail_Balance) As Sum_Avail_Balance
     ,Avg(Acc.Avail_Balance) As Avg_Avail_Balance
From   Account Acc
Group  By Acc.Product_Cd;
Query Results:
Thus, you have an evaluation:

  • There are 4 accounts taking the service "Saving deposit" (SAV) with
    the total amount of 1855.76 and each account has an average amount
    of 463.94.
  • ...

4.11- SQL Having

The HAVING clause enables you to specify conditions that filter which group results appear in the final results.
The WHERE clause places conditions on the selected columns, whereas
the HAVING clause places conditions on groups created by the GROUP BY
clause.
 
Suppose you want to group types of services (Product_Cd) on the Account
table and display types that have more than three (>3) participants.
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Select Acc.Product_Cd
     ,Count(Acc.Product_Cd) As Count_Acc
     ,Sum(Acc.Avail_Balance) As Sum_Avail_Balance -- Sum available balance
     ,Avg(Acc.Avail_Balance) As Avg_Avail_Balance -- The average available balance
From   Account Acc
Group  By Acc.Product_Cd
Having Count(Acc.Product_Cd) > 3;
Query Results:

Distinguishing Where & Having

You need to differentiate between
Where and
Having in the same statement.

  • Where is a clause used to filter data before group
  • Having is a clause used to filter data after group
If you want to have general information of a bank branch (
BRANCH Table), you can use
where to filter data before
group.
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Select Acc.Product_Cd
     ,Count(Acc.Product_Cd) As Count_Acc
     ,Sum(Acc.Avail_Balance) As Sum_Avail_Balance
     ,Avg(Acc.Avail_Balance) As Avg_Avail_Balance
From   Account Acc
-- Using WHERE to filter data before group
Where  Acc.Open_Branch_Id = 1
Group  By Acc.Product_Cd
-- Using HAVING to filter data after group
Having Count(Acc.Product_Cd) > 1;
Query Results:

5- SQL Insert

5.1- SQL Insert Into

Syntax:
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-- Syntax of Insert Into:
INSERT INTO ("column1", "column2", ...)
VALUES ("value1", "value2", ...);
For example, you insert a customer transaction in 
ACC_TRANSACTION table:
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-- Insert a record in Acc_Transaction table
-- numeric value is automatically generated for Txn_ID.
-- Now() is function of MySQL, return system datetime
-- Now(): System datetime
Insert Into Acc_Transaction
  (Txn_Id
  ,Amount
  ,Funds_Avail_Date
  ,Txn_Date
  ,Txn_Type_Cd
  ,Account_Id
  ,Execution_Branch_Id
  ,Teller_Emp_Id)
Values
  (Null-- Txn_Id (automatically generated)
  ,100 -- Amount
  ,now() -- Funds_Avail_Date
  ,now() -- Txn_Date
  ,'CDT' -- Txn_Type_Cd
  ,2 -- Account_Id
  ,Null -- Execution_Branch_Id
  ,Null -- Teller_Emp_Id
   );

5.2- SQL Insert Into Select

You can use the
Select statement to provide data for the
Insert statement. Through
I
nsert Into ... Select statement.
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-- Syntax of INSERT INTO .. SELECT
INSERT INTO "table1" ("column1", "column2", ...)
SELECT "column3", "column4", ...
FROM "table2";
Example:
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-- Insert multi record in Acc_Transaction table
-- Use Select statement to provide data.
Insert Into Acc_Transaction
 (Txn_Id
 ,Txn_Date
 ,Account_Id
 ,Txn_Type_Cd
 ,Amount
 ,Funds_Avail_Date)
 Select Null -- Txn_Id (Tự sinh ra)
       ,Acc.Open_Date -- Txn_Date
       ,Acc.Account_Id -- Account_Id
       ,'CDT' -- Txn_Type_Cd
       ,200 -- Amount
       ,Acc.Open_Date -- Funds_Avail_Date
 From   Account Acc
 Where  Acc.Product_Cd = 'CD';

6- SQL Update

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-- Syntax of Update:
UPDATE "table_name"
SET "column_1" = "new value 1", "column_2"= "new value 2"
WHERE "condition";
Update, increase the amount of money in customers' account with
CUST_ID = 1 by 2%.
Update:
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-- Update, increase the amount of money in customers' account with CUST_ID = 1 by 2%.
Update Account Acc
Set    Acc.Avail_Balance   = Acc.Avail_Balance + 2 * Acc.Avail_Balance / 100
     ,Acc.Pending_Balance = Acc.Pending_Balance +
                            2 * Acc.Pending_Balance / 100
Where  Acc.Cust_Id = 1;
Requery, after Update.

7- SQL Delete

Syntax delete records in the table
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-- Syntax delete records in the table.
DELETE FROM "table_name"
WHERE "condition";
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-- Delete two records in the Acc_Transaction table
Delete From Acc_Transaction Txn
Where  Txt.Txn_Id In (25
                   ,26);

8- SQL Functions

8.1- SQL Count

The
COUNT() function returns the number of rows that matches a specified criteria.
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-- Count the number of rows in the Account table
Select Count(Acc.Account_Id) From Account Acc;
Count the number of accounts having transaction with the bank (
Acc_Transaction Table)
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-- Count the number of accounts having transaction with the bank
Select Count(distinct txn.Account_id) From Acc_Transaction txn;

Using Group by:

A customer can open many accounts. Each account is equivalent to a service product of the bank.
You want to enumerate customers (CUST_ID) and their accounts.
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-- Count the number of accounts opened for each customer....
Select Acc.Cust_Id
     ,Count(Acc.Account_Id)
From   Account Acc
Group  By Acc.Cust_Id;

8.2- SQL Sum

The
SUM() function returns the total sum of a numeric column.
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-- Syntax:
SELECT SUM("column_name")
FROM "table_name";
Example:
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-- Find the sum of the money in customers' accounts with CUST_ID = = 1
Select Sum(Acc.Avail_Balance) From Account Acc Where Acc.Cust_Id = 1;
-- Use Group by.
-- Find the sum of the money in accounts owned by each customer.
Select Acc.Cust_Id
     ,Sum(Acc.Avail_Balance)
From   Account Acc
Group  By Acc.Cust_Id;

8.3- SQL AVG

The AVG() function returns the average value of a numeric column.
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-- Syntax:
SELECT AVG("column_name")
FROM "table_name";
Example:
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-- Find the average of money equivalent to each type of deposit.
Select Avg(Acc.Avail_Balance)
From   Account Acc
Where  Acc.Product_Cd = 'SAV';
-- Use Group by.
-- A customer can own one or more account.
-- Find the average of money in each account owned by each customer
-- (In the bank with Branch_ID = 1)
Select Acc.Cust_Id
    ,Avg(Acc.Avail_Balance)
From   Account Acc
Where  Acc.Open_Branch_Id = 1
Group  By Acc.Cust_Id;

8.4- SQL MIN

The MIN() function returns the smallest value of the selected column.
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-- Syntax:
SELECT MIN ("column_name")
FROM "table_name";
Example:
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-- Find the minimum amount of deposit.
Select Min(Acc.Avail_Balance)
From   Account Acc
Where  Acc.Product_Cd = 'SAV';
-- Use Group by.
-- A customer can own one or more account.
-- Accounts can be opened in different branches.
-- Find the amount in the account, minimum for each branch
Select Acc.Open_Branch_Id
     ,Min(Acc.Avail_Balance)
From   Account Acc
Group  By Acc.Open_Branch_Id;

8.5- SQL MAX

The
MAX() function returns the largest value of the selected column.Usage is similar to
MIN. You can refer for example to
MIN.
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-- Syntax:
SELECT MAX("column_name")
FROM "table_name";

9- SQL Join

For example, you look at the information in the 
EMPLOYEE table. You can find an employee if
you know his/her department's ID. However, it is just a meaningless
number.To find the department, you need to look for in the
DEPARTMENT table. The link of these tables to get the full information is known as
JOIN.
There are 4 ways to join two tables:
  • INNER JOIN   (JOIN)
  • LEFT OUTER JOIN  (LEFT JOIN)
  • RIGHT OUTER JOIN (RIGHT JOIN)
  • FULL OUTER JOIN    (OUTER JOIN)
  • CROSS JOIN

9.1- INNER JOIN (or JOIN)

The INNER JOIN keyword selects all rows from both tables as long as there is a match between the columns in both tables.

 

Syntax:

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-- Syntax
SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name=table2.column_name;
-- INNER JOIN can replaced by JOIN
-- Same Meaning, and result.
SELECT column_name(s)
FROM table1
JOIN table2
ON table1.column_name=table2.column_name;
Syntax:
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-- INNER JOIN 2 table: EMPLOYEE and DEPARTMENT.
Select Emp.Emp_Id
     ,Emp.First_Name
     ,Emp.Last_Name
     ,Emp.Dept_Id
     ,Dep.Name Dept_Name
From   Employee Emp
Inner  Join Department Dep
On     Emp.Dept_Id = Dep.Dept_Id
Order  By Emp.Emp_Id;

MySQL's own syntax:

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-- Other MySQL syntax to join two tables:
Select Emp.Emp_Id
  ,Emp.First_Name
  ,Emp.Last_Name
  ,Emp.Dept_Id
  ,Dep.Name Dept_Name
From   Employee   Emp
  ,Department Dep
Where  Emp.Dept_Id = Dep.Dept_Id
Order  By Emp.Emp_Id;

9.2- LEFT OUTER JOIN (or LEFT JOIN)

The
LEFT OUTER JOIN keyword returns all rows from
the left table (table1), with the matching rows in the right table
(table2). The result is NULL in the right side when there is no match.
See the illustration below:
Example:
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-- Customer LEFT OUTER JOIN Officer
-- LEFT OUTER JOIN Can be replaced by LEFT JOIN (meaning, and the result is the same)
Select Cus.Cust_Id
     ,Cus.Fed_Id
     ,Cus.State
     ,Ofc.Cust_Id As Ofc_Cust_Id
     ,Ofc.Officer_Id
     ,Ofc.Start_Date
     ,Ofc.Title
From   Customer Cus  -- Table1
Left   Outer Join Officer Ofc  -- Table2
On     Cus.Cust_Id = Ofc.Cust_Id;
Result:

9.3- RIGHT OUTER JOIN (or RIGHT JOIN)

RIGHT OUTER JOIN is quite similar to
LEFT OUTER JOIN:

9.4- FULL OUTER JOIN (or OUTER JOIN)

FULL OUTER JOIN is a combination of
LEFT OUTER JOIN and
RIGHT OUTER JOIN
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-- Syntax:  (FULL OUTER JOIN)
-- Or: FULL JOIN
SELECT columns
FROM table1
FULL [OUTER] JOIN table2
ON table1.column = table2.column;

10- Subquery

In
MySQL, a subquery is a query within a query. You can create subqueries within your SQL statements. These subqueries can reside in the
WHERE clause, the
FROM clause, or the
SELECT clause.

10.1- Subquery in the Where clause

Most often, the subquery will be found in the
WHERE clause. These subqueries are also called
nested subqueries.
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Select Acc.Account_Id
     ,Acc.Open_Date
     ,Acc.Product_Cd
     ,Acc.Avail_Balance
From   Account Acc
Where  Acc.Cust_Id In
      (Select Cus.Cust_Id From Customer Cus Where Cus.Cust_Type_Cd = 'B');

10.2- Subquery in the From clause

A subquery can also be found in the
FROM clause. These are called
inline views.
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Select Cus.Cust_Id
     ,Cus.Address
     ,Cus.Fed_Id
     ,Acc2.Sum_Avail_Balance
From   Customer Cus
     , -- Define a inline view.
      (Select Acc.Cust_Id
             ,Sum(Acc.Avail_Balance) As Sum_Avail_Balance
       From   Account Acc
       Group  By Acc.Cust_Id) Acc2
Where  Cus.Cust_Id = Acc2.Cust_Id;

10.3- Subquery in the Select clause

A subquery can also be found in the SELECT clause.
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Select Cus.Cust_Id
     ,Cus.Address
     ,Cus.Fed_Id
     ,(Select Sum(Acc.Avail_Balance)
       From   Account Acc
       Where  Acc.Cust_Id = Cus.Cust_Id) As Sum_Avail_Balance
From   Customer Cus;
The trick to placing a subquery in the
select clause is that the subquery must return a single value. This is why an aggregate function such as
SUM function,
COUNT function,
MIN function, or
MAX function is commonly used in the subquery. 

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