knowledge/technology/dev/programming/languages/SQL.md

---
obj: concept
---
# SQL
Structured Query Language (SQL) is a powerful and standardized programming language used for managing and manipulating relational databases. It allows users to interact with databases to create, retrieve, update, and delete data, as well as define and modify the database schema. SQL is widely used in web development, data analysis, and various other domains where data storage and retrieval are essential.

A database most often contains one or more tables. Each table is identified by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with data.

# SQL Statements
Most of the actions you need to perform on a database are done with SQL statements.
Example:
```sql
SELECT * FROM Customers;
```

### Comments
Comments are used to explain sections of SQL statements, or to prevent execution of SQL statements.

Single Line Comments:
```sql
--Select all:
SELECT * FROM Customers;
```

Multi Line comments:
```sql
/*Select all the columns
of all the records
in the Customers table:*/
SELECT * FROM Customers;
```

## SELECT
The `SELECT` statement is used to select data from a database.

Select:
```sql
SELECT column1, column2, ...  
FROM table_name;
```

Select all:
```sql
SELECT * FROM table
```

### SELECT DISTINCT
The `SELECT DISTINCT` statement is used to return only distinct (different) values.

```sql
SELECT DISTINCT Country FROM Customers;
SELECT COUNT(DISTINCT Country) FROM Customers;
```

## WHERE
The `WHERE` clause is used to filter records.

```sql
SELECT column1, column2, ...
FROM table_name
WHERE condition;
```

```sql
SELECT * FROM Customers
WHERE Country='Mexico';
```

```sql
SELECT * FROM Customers
WHERE CustomerID=1;
```

The following operators can be used in the `WHERE` clause:

| Operator | Description                                                                     |
| -------- | ------------------------------------------------------------------------------- |
| =        | Equal                                                                           |
| >        | Greater than                                                                    |
| <        | Less than                                                                       |
| >=       | Greater than or equal                                                           |
| <=       | Less than or equal                                                              |
| <>       | Not equal. **Note:** In some versions of SQL this operator may be written as != |
| BETWEEN  | Between a certain range                                                         |
| LIKE     | Search for a pattern                                                            |
| IN       | To specify multiple possible values for a column                                |

### LIKE
The `LIKE` operator is used in a `WHERE` clause to search for a specified pattern in a column.

There are two wildcards often used in conjunction with the `LIKE` operator:
	-  The percent sign `%` represents zero, one, or multiple characters
	-  The underscore sign `_` represents one, single character

```sql
SELECT column1, column2, ...
FROM table_name
WHERE columnN LIKE pattern;
```

#### The _ Wildcard
The `_` wildcard represents a single character.
It can be any character or number, but each `_` represents one, and only one, character.

```sql
SELECT * FROM Customers  
WHERE city LIKE 'L_nd__';
```

#### The % Wildcard
The `%` wildcard represents any number of characters, even zero characters.

```sql
SELECT * FROM Customers  
WHERE city LIKE '%L%';
```

#### Starts With
```sql
SELECT * FROM Customers  
WHERE CustomerName LIKE 'La%';
```

#### Ends With
```sql
SELECT * FROM Customers  
WHERE CustomerName LIKE '%a';
```

#### Contains
```sql
SELECT * FROM Customers  
WHERE CustomerName LIKE '%or%';
```

### IN
The `IN` operator allows you to specify multiple values in a `WHERE` clause.

The `IN` operator is a shorthand for multiple `OR` conditions.

```sql
SELECT * FROM Customers  
WHERE Country IN ('Germany', 'France', 'UK');
```

Subquery:
```sql
SELECT * FROM Customers  
WHERE CustomerID NOT IN (SELECT CustomerID FROM Orders);
```

### BETWEEN
The `BETWEEN` operator selects values within a given range. The values can be numbers, text, or dates.

The `BETWEEN` operator is inclusive: begin and end values are included.

```sql
SELECT column_name(s)
FROM table_name
WHERE column_name BETWEEN value1 AND value2;

SELECT * FROM Orders  
WHERE OrderDate BETWEEN '1996-07-01' AND '1996-07-31';
```

### AND
The `WHERE` clause can contain one or many `AND` operators.

```sql
SELECT *
FROM Customers
WHERE Country = 'Spain' AND CustomerName LIKE 'G%';
```

### OR
The `WHERE` clause can contain one or more `OR` operators.

```sql
SELECT *
FROM Customers
WHERE Country = 'Germany' OR Country = 'Spain';
```

### Combine AND & OR
```sql
SELECT * FROM Customers
WHERE Country = 'Spain' AND (CustomerName LIKE 'G%' OR CustomerName LIKE 'R%');
```

### NOT
The `NOT` operator is used in combination with other operators to give the opposite result, also called the negative result.

```sql
SELECT * FROM Customers  
WHERE NOT Country = 'Spain';

SELECT * FROM Customers  
WHERE City NOT IN ('Paris', 'London');
```

## ORDER BY
The `ORDER BY` keyword is used to sort the result-set in ascending or descending order.

The `ORDER BY` keyword sorts the records in ascending order by default. To sort the records in descending order, use the `DESC` keyword.

```sql
SELECT column1, column2, ...
FROM table_name
ORDER BY column1, column2, ... ASC|DESC;
```

## INSERT INTO
The `INSERT INTO` statement is used to insert new records in a table.

Example:
```sql
INSERT INTO table_name (column1, column2, column3, ...)
VALUES (value1, value2, value3, ...);
```

Insert Many:
```sql
INSERT INTO Customers (CustomerName, ContactName, Address, City, PostalCode, Country)
VALUES
('Cardinal', 'Tom B. Erichsen', 'Skagen 21', 'Stavanger', '4006', 'Norway'),
('Greasy Burger', 'Per Olsen', 'Gateveien 15', 'Sandnes', '4306', 'Norway'),
('Tasty Tee', 'Finn Egan', 'Streetroad 19B', 'Liverpool', 'L1 0AA', 'UK');
```

## UPDATE
The `UPDATE` statement is used to modify the existing records in a table.

```sql
UPDATE table_name
SET column1 = value1, column2 = value2, ...
WHERE condition;
```

## DELETE
The `DELETE` statement is used to delete existing records in a table.

Example:
```sql
DELETE FROM table_name WHERE condition;
```

Delete All:
```sql
DELETE FROM table_name;
```

Delete Table:
```sql
DROP TABLE table_name;
```

## LIMIT
The `LIMIT` clause is used to specify the number of records to return.

```sql
SELECT * FROM Customers
LIMIT 3;
```

## Aliases
SQL aliases are used to give a table, or a column in a table, a temporary name.
Aliases are often used to make column names more readable.
An alias only exists for the duration of that query.
An alias is created with the `AS` keyword.

```sql
SELECT column_name AS alias_name
FROM table_name;
```

## JOIN
A `JOIN` clause is used to combine rows from two or more tables, based on a related column between them.

### INNER JOIN
The `INNER JOIN` keyword selects records that have matching values in both tables.

```sql
SELECT Orders.OrderID, Customers.CustomerName  
FROM Orders  
INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID;
```

# Database
## Create Database
The `CREATE DATABASE` statement is used to create a new SQL database.

```sql
CREATE DATABASE databasename;
```

## Delete Database
The `DROP DATABASE` statement is used to drop an existing SQL database.

```sql
DROP DATABASE databasename;
```

## Create Table
The `CREATE TABLE` statement is used to create a new table in a database.

```sql
CREATE TABLE table_name (
    column1 datatype,
    column2 datatype,
    column3 datatype,
   ....
);

CREATE TABLE Persons (
    PersonID int,
    LastName varchar(255),
    FirstName varchar(255),
    Address varchar(255),
    City varchar(255)
);
```

## Delete Table
The `DROP TABLE` statement is used to drop an existing table in a database.

```sql
DROP TABLE table_name;
```

## Change Table
The `ALTER TABLE` statement is used to add, delete, or modify columns in an existing table.
The `ALTER TABLE` statement is also used to add and drop various constraints on an existing table.

Add Column:
```sql
ALTER TABLE Customers
ADD Email varchar(255);
```

Drop Column:
```sql
ALTER TABLE Customers  
DROP COLUMN Email;
```

Rename Column:
```sql
ALTER TABLE table_name
RENAME COLUMN old_name to new_name;
```

Change Datatype:
```sql
ALTER TABLE Persons
ALTER COLUMN DateOfBirth year;
```

## Constraints
Constraints can be specified when the table is created with the `CREATE TABLE` statement, or after the table is created with the `ALTER TABLE` statement.

```sql
CREATE TABLE table_name (
    column1 datatype constraint,
    column2 datatype constraint,
    column3 datatype constraint,
    ....
);
```

The following constraints are commonly used in SQL:
### NOT NULL
By default, a column can hold NULL values.

The `NOT NULL` constraint enforces a column to NOT accept NULL values.

This enforces a field to always contain a value, which means that you cannot insert a new record, or update a record without adding a value to this field.

```sql
CREATE TABLE Persons (  
    ID int NOT NULL,  
    LastName varchar(255) NOT NULL,  
    FirstName varchar(255) NOT NULL,  
    Age int  
);
```

### UNIQUE
The `UNIQUE` constraint ensures that all values in a column are different.

Both the `UNIQUE` and `PRIMARY KEY` constraints provide a guarantee for uniqueness for a column or set of columns.

A `PRIMARY KEY` constraint automatically has a `UNIQUE` constraint.

However, you can have many `UNIQUE` constraints per table, but only one `PRIMARY KEY` constraint per table.

### PRIMARY KEY
The `PRIMARY KEY` constraint uniquely identifies each record in a table.

Primary keys must contain UNIQUE values, and cannot contain NULL values.

A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).

```sql
CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    PRIMARY KEY (ID)
);
```

### FOREIGN KEY
The `FOREIGN KEY` constraint is used to prevent actions that would destroy links between tables.

A `FOREIGN KEY` is a field (or collection of fields) in one table, that refers to the `PRIMARY KEY` in another table.

The table with the foreign key is called the child table, and the table with the primary key is called the referenced or parent table.

```sql
CREATE TABLE Orders (
    OrderID int NOT NULL,
    OrderNumber int NOT NULL,
    PersonID int,
    PRIMARY KEY (OrderID),
    FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);
```

### CHECK
The `CHECK` constraint is used to limit the value range that can be placed in a column.

If you define a `CHECK` constraint on a column it will allow only certain values for this column.

If you define a `CHECK` constraint on a table it can limit the values in certain columns based on values in other columns in the row.

```sql
CREATE TABLE Persons (  
    ID int NOT NULL,  
    LastName varchar(255) NOT NULL,  
    FirstName varchar(255),  
    Age int,  
    CHECK (Age>=18)  
);
```

### DEFAULT
The `DEFAULT` constraint is used to set a default value for a column.

The default value will be added to all new records, if no other value is specified.

```sql
CREATE TABLE Orders (  
    ID int NOT NULL,  
    OrderNumber int NOT NULL,  
    OrderDate date DEFAULT GETDATE()  
);
```

### AUTO_INCREMENT
Auto-increment allows a unique number to be generated automatically when a new record is inserted into a table.

Often this is the primary key field that we would like to be created automatically every time a new record is inserted.

```sql
CREATE TABLE Persons (  
    Personid int NOT NULL AUTO_INCREMENT,  
    LastName varchar(255) NOT NULL,  
    FirstName varchar(255),  
    Age int,  
    PRIMARY KEY (Personid)  
);
```

## Create Index
The `CREATE INDEX` statement is used to create indexes in tables.

Indexes are used to retrieve data from the database more quickly than otherwise. The users cannot see the indexes, they are just used to speed up searches/queries.

```sql
CREATE INDEX index_name
ON table_name (column1, column2, ...);
```

## Dates
**MySQL** comes with the following data types for storing a date or a date/time value in the database:
	- `DATE` - format YYYY-MM-DD
	- `DATETIME` - format: YYYY-MM-DD HH:MI:SS
	- `TIMESTAMP` - format: YYYY-MM-DD HH:MI:SS
	- `YEAR` - format YYYY or YY

## Data Types
### String
| Data type                   | Description                                                                                                                                                                                                                                                             |
| --------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| CHAR(size)                  | A FIXED length string (can contain letters, numbers, and special characters). The _size_ parameter specifies the column length in characters - can be from 0 to 255. Default is 1                                                                                       |
| VARCHAR(size)               | A VARIABLE length string (can contain letters, numbers, and special characters). The _size_ parameter specifies the maximum string length in characters - can be from 0 to 65535                                                                                        |
| BINARY(size)                | Equal to CHAR(), but stores binary byte strings. The _size_ parameter specifies the column length in bytes. Default is 1                                                                                                                                                |
| VARBINARY(size)             | Equal to VARCHAR(), but stores binary byte strings. The _size_ parameter specifies the maximum column length in bytes.                                                                                                                                                  |
| TINYBLOB                    | For BLOBs (Binary Large Objects). Max length: 255 bytes                                                                                                                                                                                                                 |
| TINYTEXT                    | Holds a string with a maximum length of 255 characters                                                                                                                                                                                                                  |
| TEXT(size)                  | Holds a string with a maximum length of 65,535 bytes                                                                                                                                                                                                                    |
| BLOB(size)                  | For BLOBs (Binary Large Objects). Holds up to 65,535 bytes of data                                                                                                                                                                                                      |
| MEDIUMTEXT                  | Holds a string with a maximum length of 16,777,215 characters                                                                                                                                                                                                           |
| MEDIUMBLOB                  | For BLOBs (Binary Large Objects). Holds up to 16,777,215 bytes of data                                                                                                                                                                                                  |
| LONGTEXT                    | Holds a string with a maximum length of 4,294,967,295 characters                                                                                                                                                                                                        |
| LONGBLOB                    | For BLOBs (Binary Large Objects). Holds up to 4,294,967,295 bytes of data                                                                                                                                                                                               |
| ENUM(val1, val2, val3, ...) | A string object that can have only one value, chosen from a list of possible values. You can list up to 65535 values in an ENUM list. If a value is inserted that is not in the list, a blank value will be inserted. The values are sorted in the order you enter them |
| SET(val1, val2, val3, ...)  | A string object that can have 0 or more values, chosen from a list of possible values. You can list up to 64 values in a SET list                                                                                                                                       |

### Numeric
| Data type                     | Description                                                                                                                                                                                                                                                                                                |
| ----------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| BIT(_size_)                   | A bit-value type. The number of bits per value is specified in _size_. The _size_ parameter can hold a value from 1 to 64. The default value for _size_ is 1.                                                                                                                                              |
| TINYINT(_size_)               | A very small integer. Signed range is from -128 to 127. Unsigned range is from 0 to 255. The _size_ parameter specifies the maximum display width (which is 255)                                                                                                                                           |
| BOOL                          | Zero is considered as false, nonzero values are considered as true.                                                                                                                                                                                                                                        |
| BOOLEAN                       | Equal to BOOL                                                                                                                                                                                                                                                                                              |
| SMALLINT(_size_)              | A small integer. Signed range is from -32768 to 32767. Unsigned range is from 0 to 65535. The _size_ parameter specifies the maximum display width (which is 255)                                                                                                                                          |
| MEDIUMINT(_size_)             | A medium integer. Signed range is from -8388608 to 8388607. Unsigned range is from 0 to 16777215. The _size_ parameter specifies the maximum display width (which is 255)                                                                                                                                  |
| INT(_size_)                   | A medium integer. Signed range is from -2147483648 to 2147483647. Unsigned range is from 0 to 4294967295. The _size_ parameter specifies the maximum display width (which is 255)                                                                                                                          |
| INTEGER(_size_)               | Equal to INT(size)                                                                                                                                                                                                                                                                                         |
| BIGINT(_size_)                | A large integer. Signed range is from -9223372036854775808 to 9223372036854775807. Unsigned range is from 0 to 18446744073709551615. The _size_ parameter specifies the maximum display width (which is 255)                                                                                               |
| FLOAT(_size_, _d_)            | A floating point number. The total number of digits is specified in _size_. The number of digits after the decimal point is specified in the _d_ parameter. This syntax is deprecated in MySQL 8.0.17, and it will be removed in future MySQL versions                                                     |
| FLOAT(_p_)                    | A floating point number. MySQL uses the _p_ value to determine whether to use FLOAT or DOUBLE for the resulting data type. If _p_ is from 0 to 24, the data type becomes FLOAT(). If _p_ is from 25 to 53, the data type becomes DOUBLE()                                                                  |
| DOUBLE(_size_, _d_)           | A normal-size floating point number. The total number of digits is specified in _size_. The number of digits after the decimal point is specified in the _d_ parameter                                                                                                                                     |
| DOUBLE PRECISION(_size_, _d_) |                                                                                                                                                                                                                                                                                                            |
| DECIMAL(_size_, _d_)          | An exact fixed-point number. The total number of digits is specified in _size_. The number of digits after the decimal point is specified in the _d_ parameter. The maximum number for _size_ is 65. The maximum number for _d_ is 30. The default value for _size_ is 10. The default value for _d_ is 0. |
| DEC(_size_, _d_)              | Equal to DECIMAL(size,d)                                                                                                                                                                                                                                                                                   |

### Date & Time
| Data type        | Description                                                                                                                                                                                                                                                                                                                                                                                                       |
| ---------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| DATE             | A date. Format: YYYY-MM-DD. The supported range is from '1000-01-01' to '9999-12-31'                                                                                                                                                                                                                                                                                                                              |
| DATETIME(_fsp_)  | A date and time combination. Format: YYYY-MM-DD hh:mm:ss. The supported range is from '1000-01-01 00:00:00' to '9999-12-31 23:59:59'. Adding DEFAULT and ON UPDATE in the column definition to get automatic initialization and updating to the current date and time                                                                                                                                             |
| TIMESTAMP(_fsp_) | A timestamp. TIMESTAMP values are stored as the number of seconds since the Unix epoch ('1970-01-01 00:00:00' UTC). Format: YYYY-MM-DD hh:mm:ss. The supported range is from '1970-01-01 00:00:01' UTC to '2038-01-09 03:14:07' UTC. Automatic initialization and updating to the current date and time can be specified using DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP in the column definition |
| TIME(_fsp_)      | A time. Format: hh:mm:ss. The supported range is from '-838:59:59' to '838:59:59'                                                                                                                                                                                                                                                                                                                                 |
| YEAR             | A year in four-digit format. Values allowed in four-digit format: 1901 to 2155, and 0000.  <br>MySQL 8.0 does not support year in two-digit format.                                                                                                                                                                                                                                                               |




# Functions

## String Functions
### CHARACTER_LENGTH()
The `CHARACTER_LENGTH()` function return the length of a string (in characters).

```sql
SELECT CHARACTER_LENGTH("SQL Tutorial") AS LengthOfString;
```

### CONCAT()
The `CONCAT()` function returns a concat string of parameters.

```sql
SELECT CONCAT(Name, LastName)
FROM persons;
```

### LOWER() & UPPER()
The `UPPER()` function converts a string to upper-case.
The `LOWER()` function converts a string to lower-case.

```sql
SELECT LOWER("SQL");
SELECT UPPER("SQL");
```

### TRIM() (RTRIM() & LTRIM())
The `TRIM()` function removes leading and trailing spaces from a string. `LTRIM()`  & `RTRIM()` remove leading and trailing spaces from the left or right respectively.

```sql
SELECT TRIM('    SQL    ') AS TrimmedString;
```

### SUBSTRING()
The SUBSTRING() function extracts a substring from a string (starting at any position).

```sql
SUBSTRING(string, start, length)
```

### LPAD() & RPAD()
The `LPAD()` & `RPAD()` function left- or right-pads a string with another string, to a certain length.

```sql
RPAD(string, length, rpad_string)
```

### LEFT() & RIGHT()
These functions extract a number of characters from a string. (From left or right respectively)

```sql
RIGHT(string, number_of_chars)
```


### REVERSE()
The `REVERSE()` function reverses a string and returns the result.

### REPLACE()
The `REPLACE()` function replaces all occurrences of a substring within a string, with a new substring.

```sql
REPLACE(string, from_string, new_string)
```

 ### REPEAT()
 The `REPEAT()` function repeats a string as many times as specified.

```sql
REPEAT(string, number)
```

## Numeric Functions
### MIN() & MAX()
The `MIN()` function returns the smallest value of the selected column.

The `MAX()` function returns the largest value of the selected column.

```sql
SELECT MIN(Price)  
FROM Products;

SELECT MAX(Price)  
FROM Products;
```

### COUNT()
The `COUNT()` function returns the number of rows that matches a specified criterion.

```sql
SELECT COUNT(*)  
FROM Products;
```

### SUM()
The `SUM()` function returns the total sum of a numeric column.

Example:
```sql
SELECT SUM(Quantity) AS total  
FROM OrderDetails;
```

Expressions:
```sql
SELECT SUM(Quantity * 10)  
FROM OrderDetails;
```

### AVG()
The `AVG()` function returns the average value of a numeric column.

```sql
SELECT AVG(Price)  
FROM Products;

SELECT * FROM Products  
WHERE price > (SELECT AVG(price) FROM Products);
```

## Date Functions

### Extract information
1. **DAY()**: Returns the day of the month for a given date.
2. **HOUR()**: Returns the hour part for a given date.
3. **MINUTE()**: Returns the minute part of a time/datetime.
4. **SECOND()**: Returns the seconds part of a time/datetime.
5. **MICROSECOND()**: Returns the microsecond part of a time/datetime.
6. **DAYNAME()**: Returns the weekday name for a given date.
7. **MONTH()**: Returns the month for a given date
8. **MONTHNAME()**: Returns the name of the month for a given date.
8. **DAYOFMONTH()**: Returns the day of the month for a given date.
9. **DAYOFWEEK()**: Returns the weekday index for a given date.
10. **DAYOFYEAR()**: Returns the day of the year for a given date.
11. **QUARTER()**: Returns the quarter of the year for a given date value.
12. **WEEKDAY()**: Returns the weekday number for a given date.
13. **WEEKOFYEAR()**: Returns the week number for a given date.

### Create Datetimes
Current date and time:
```sql
NOW()
```