SQL Fundamentals: Using Single-Row Functions to Customize Output使用单行函数自定义输出

SQL Fundamentals || Oracle SQL语言

DUAL is a public table that you can use to view results from functions and calculations.

The DUAL table is owned by the user SYS and can be accessed by all users.

It contains one column, DUMMY, and one row with the value X.

SQL Fundamentals: Using Single-Row Functions to Customize Output使用单行函数自定义输出

SQL Fundamentals || Single-Row Functions || 字符函数 character functions

SQL Fundamentals || Single-Row Functions || 数字函数number functions

SQL Fundamentals || Single-Row Functions || 日期函数date functions

SQL Fundamentals || Single-Row Functions || 转换函数 Conversion function

SQL Fundamentals || Single-Row Functions || 通用函数 General function

Using Single-Row Functions to Customize Output

Oracle SQL supplies a rich library of in-built functions which can be employed for various tasks. The essential capabilities of a functions can be the case conversion of strings, in-string or substring operations, mathematical computations on numeric data, and date operations on date type values. SQL Functions optionally take arguments from the user and mandatorily return a value.

• Describe various types of functions available in SQL
• Use character, number, and date functions in SELECT statements

字符、数字、日期函数

1、SQL Functions

Functions are a very powerful feature of SQL. They can be used to do the following:

Perform calculations on data	执行数据计算
Modify individual data items	修改单独的数据项
Manipulate output for groups of rows	操纵行组的输出
Format dates and numbers for display	格式化日期和数字进行显示
Convert column data types	转换列数据类型

SQL functions sometimes take arguments and always return a value.

SQL有时候接收参数并总是返回一个值.

2、Two Types of SQL Function：

Single-row functions	单行函数 These functions operate on single rows only and return one result per row. 单行函数只操作单个行并为每一行返回一个结果. - Single row functions are the one who work on single row and return one output per row. For example, length and case conversion functions are single row functions.
Multiple-row functions	多行函数 - Multiple row functions work upon group of rows and return one result for the complete set of rows. They are also known as Group Functions.

3、Single-row functions单行函数

Single row functions

Single row functions can be character functions, numeric functions, date functions, and conversion functions. Note that these functions are used to manipulate data items. These functions require one or more input arguments and operate on each row, thereby returning one output value for each row. Argument can be a column, literal or an expression. Single row functions can be used in SELECT statement, WHERE and ORDER BY clause.

Single-row functions are used to manipulate data items. They accept one or more arguments and return one value for each row that is returned by the query.

（1）特点

Manipulate data items	操作数据项
Accept arguments and return one value	接收参数并返回一个值
Act on each row that is returned	每一行进行操作
Return one result per row	每一行返回一个值
Many modify the data type	单行函数可以修改数据类型
Can be nested	单行函数可以嵌套
Accept arguments that can be a column or an expression An argument can be one of the following: User-supplied constant Variable value Column name expression	函数接收的参数可以是列名或者表达式 用户提供的常量 变量值 列名 表达式

(2)、语法

Function_name [(arg1,arg2….)]

（3）、类型

字符函数 character functions	Accept character input and can return both character and number values.
数字函数 number functions	Accept numeric input and return numeric values.
日期函数 date functions	Operate on values of the DATE data type (All date functions return a value of the DATE data type except the MONTHS_BETWEEN function, which returns a number) 所有日期函数都返回一个DATE类型的值，除了MONTHS_BETWEEN函数，它返回一个数字.
转换函数 Conversion function	Convert a value from one data type to another
通用函数 General function	General functions - Usually contains NULL handling functions. The functions under the category are NVL, NVL2, NULLIF, COALESCE, CASE, DECODE. NVL：对空值做处理 NVL2：对空值做处理 NULLIF：对空值做处理 COALESCE CASE DECODE General functions The SELECT query below demonstrates the use of NVL function. SELECT first_name, last_name, salary, NVL (commission_pct,0) FROM employees WHERE rownum < 5; FIRST_NAME           LAST_NAME                     SALARY NVL(COMMISSION_PCT,0) -------------------- ------------------------- ---------- --------------------- Steven               King                           24000                     0 Neena                Kochhar                        17000                     0 Lex                  De Haan                        17000                     0 Alexander            Hunold                          9000                     0