SQL 方法

SQL has many built-in functions that enable you to perform calculations on data.

## SQL Aggregate Functions
SQL aggregate functions perform calculation on a set of values and return a single value. The following table summarizes some useful aggregate functions:

| Function | Description                                |
|----------|--------------------------------------------|
| AVG()    | Returns the average of values              |
| SUM()    | Returns the sum of values                  |
| COUNT()  | Returns the number of rows in a result set |
| MAX()    | Returns the maximum value                  |
| MIN()    | Returns the minimum value                  |

## SQL String Functions
SQL string functions perform operation on a string input value and return a string or numeric value. The following table summarizes some useful string functions:

| Function    | Description                                                                   |
|-------------|-----------------------|
| CONCAT()    | Returns a string by concatenating two or more string values. |
| CONCAT_WS() | Returns a string by concatenating two or more string values with a separator. |
| FORMAT()    | Returns a value formatted with the specified format. |
| LOWER()     | Converts a string to lowercase. |
| UPPER()     | Converts a string to uppercase. |
| TRIM()      | Remove leading and trailing spaces from a string. |
| REVERSE()   | Returns the reverse order of a string value. |
| SUBSTRING() | Returns a substring from string. |

## SQL Date Functions
Date functions are used to manipulate or perform operation on date values.

### MySQL Date Functions
The following table summarizes some most important MySQL's built-in date functions:

| Function      | Description                                                       |
|---------------|-------------------------------------------------------------------|
| NOW()         | Returns the current date and time.                                |
| CURDATE()     | Returns the current date.                                         |
| CURTIME()     | Returns the current time                                          |
| DATE()        | Extract the date part of a date or datetime expression.           |
| DAY()         | Returns the day of the month (0-31).                              |
| DAYNAME()     | Returns the name of the weekday.                                  |
| MONTH()       | Returns the month from the date passed (1-12).                    |
| MONTHNAME()   | Returns the name of the month.                                    |
| YEAR()        | Returns the year.                                                 |
| DATE_FORMAT() | Displays date and time value in other formats.                    |
| EXTRACT()     | Extract part of a date.                                           |
| DATE_ADD()    | Adds a specified time value (or interval) to a date value.        |
| DATE_SUB()    | Subtracts a specified time value (or interval) from a date value. |
| DATEDIFF()    | Returns the number of days between two dates                      |

<div class="callout callout-info mb-3">Note: The date and time returned by these functions is the date and time of the computer on which the instance of database server is running.</div>

### SQL Server Date Functions
The following table summarizes some most important SQL Server's built-in date functions:

| Function   | Description                |
|------------|----------------------------|
| GETDATE()  | Returns the current date and time. |
| DATEPART() | Returns the specified datepart of the specified date e.g. DATEPART(year,'2016-10-25') return 2016. |
| DAY()      | Returns the day of the month (0-31). |
| MONTH()    | Returns the month from the specified date (0-12). |
| YEAR()     | Returns the year from the specified date. |
| DATEADD()  | Adds or subtracts a specified time interval from a date. |
| DATEDIFF() | Returns the date or time between two specified dates. |
| CONVERT()  | Displays date and time value in other formats. |