本文將分享15個初學者必看的基礎SQL查詢語句，都很基礎，但是你不一定都會，所以好好看看吧。
1、創建表和數據插入SQL
我們在開始創建數據表和向表中插入演示數據之前，我想給大家解釋一下實時數據表的設計理念，這樣也許能幫助大家能更好的理解SQL查詢。
在數據庫設計中，有一條非常重要的規則就是要正確建立主鍵和外鍵的關系。
現在我們來創建幾個餐廳訂單管理的數據表，一共用到3張數據表，Item Master表、Order Master表和Order Detail表。
創建表：
創建Item Master表：
CREATE TABLE [dbo].[ItemMasters]( [Item_Code] [varchar](20) NOT NULL, [Item_Name] [varchar](100) NOT NULL, [Price] Int NOT NULL, [TAX1] Int NOT NULL, [Discount] Int NOT NULL, [Description] [varchar](200) NOT NULL, [IN_DATE] [datetime] NOT NULL, [IN_USR_ID] [varchar](20) NOT NULL, [UP_DATE] [datetime] NOT NULL, [UP_USR_ID] [varchar](20) NOT NULL, CONSTRAINT [PK_ItemMasters] PRIMARY KEY CLUSTERED ( [Item_Code] ASC)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]) ON [PRIMARY]
向Item Master表插入數據：
INSERT INTO [ItemMasters] ([Item_Code],[Item_Name],[Price],[TAX1],[Discount],[Description],[IN_DATE] ,[IN_USR_ID],[UP_DATE],[UP_USR_ID]) VALUES ('Item001','Coke',55,1,0,'Coke which need to be cold',GETDATE(),'SHANU' ,GETDATE(),'SHANU')INSERT INTO [ItemMasters] ([Item_Code],[Item_Name],[Price],[TAX1],[Discount],[Description],[IN_DATE] ,[IN_USR_ID],[UP_DATE],[UP_USR_ID]) VALUES ('Item002','Coffee',40,0,2,'Coffe Might be Hot or Cold user choice',GETDATE(),'SHANU' ,GETDATE(),'SHANU')INSERT INTO [ItemMasters] ([Item_Code],[Item_Name],[Price],[TAX1],[Discount],[Description],[IN_DATE] ,[IN_USR_ID],[UP_DATE],[UP_USR_ID]) VALUES ('Item003','Chiken Burger',125,2,5,'Spicy',GETDATE(),'SHANU' ,GETDATE(),'SHANU')INSERT INTO [ItemMasters] ([Item_Code],[Item_Name],[Price],[TAX1],[Discount],[Description],[IN_DATE] ,[IN_USR_ID],[UP_DATE],[UP_USR_ID]) VALUES ('Item004','Potato Fry',15,0,0,'No Comments',GETDATE(),'SHANU' ,GETDATE(),'SHANU')創建Order Master表：
CREATE TABLE [dbo].[OrderMasters]( [Order_No] [varchar](20) NOT NULL, [Table_ID] [varchar](20) NOT NULL, [Description] [varchar](200) NOT NULL, [IN_DATE] [datetime] NOT NULL, [IN_USR_ID] [varchar](20) NOT NULL, [UP_DATE] [datetime] NOT NULL, [UP_USR_ID] [varchar](20) NOT NULL, CONSTRAINT [PK_OrderMasters] PRIMARY KEY CLUSTERED ( [Order_No] ASC)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]) ON [PRIMARY]
向Order Master表插入數據：
INSERT INTO [OrderMasters] ([Order_No],[Table_ID] ,[Description],[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID]) VALUES ('Ord_001','T1','',GETDATE(),'SHANU' ,GETDATE(),'SHANU')INSERT INTO [OrderMasters] ([Order_No],[Table_ID] ,[Description],[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID]) VALUES ('Ord_002','T2','',GETDATE(),'Mak' ,GETDATE(),'MAK')INSERT INTO [OrderMasters] ([Order_No],[Table_ID] ,[Description],[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID]) VALUES ('Ord_003','T3','',GETDATE(),'RAJ' ,GETDATE(),'RAJ')創建Order Detail表：
CREATE TABLE [dbo].[OrderDetails]( [Order_Detail_No] [varchar](20) NOT NULL, [Order_No] [varchar](20) CONSTRAINT fk_OrderMasters FOREIGN KEY REFERENCES OrderMasters(Order_No), [Item_Code] [varchar](20) CONSTRAINT fk_ItemMasters FOREIGN KEY REFERENCES ItemMasters(Item_Code), [Notes] [varchar](200) NOT NULL, [QTY] INT NOT NULL, [IN_DATE] [datetime] NOT NULL, [IN_USR_ID] [varchar](20) NOT NULL, [UP_DATE] [datetime] NOT NULL, [UP_USR_ID] [varchar](20) NOT NULL, CONSTRAINT [PK_OrderDetails] PRIMARY KEY CLUSTERED ( [Order_Detail_No] ASC)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]) ON [PRIMARY]--Now let’s insert the 3 items for the above Order No 'Ord_001'.INSERT INTO [OrderDetails] ([Order_Detail_No],[Order_No],[Item_Code],[Notes],[QTY] ,[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID]) VALUES ('OR_Dt_001','Ord_001','Item001','Need very Cold',3 ,GETDATE(),'SHANU' ,GETDATE(),'SHANU')INSERT INTO [OrderDetails] ([Order_Detail_No],[Order_No],[Item_Code],[Notes],[QTY] ,[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID]) VALUES ('OR_Dt_002','Ord_001','Item004','very Hot ',2 ,GETDATE(),'SHANU' ,GETDATE(),'SHANU')INSERT INTO [OrderDetails] ([Order_Detail_No],[Order_No],[Item_Code],[Notes],[QTY] ,[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID]) VALUES ('OR_Dt_003','Ord_001','Item003','Very Spicy',4 ,GETDATE(),'SHANU' ,GETDATE(),'SHANU')向Order Detail表插入數據：
INSERT INTO [OrderDetails] ([Order_Detail_No],[Order_No],[Item_Code],[Notes],[QTY] ,[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID]) VALUES ('OR_Dt_004','Ord_002','Item002','Need very Hot',2 ,GETDATE(),'SHANU' ,GETDATE(),'SHANU')INSERT INTO [OrderDetails] ([Order_Detail_No],[Order_No],[Item_Code],[Notes],[QTY] ,[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID]) VALUES ('OR_Dt_005','Ord_002','Item003','very Hot ',2 ,GETDATE(),'SHANU' ,GETDATE(),'SHANU')INSERT INTO [OrderDetails] ([Order_Detail_No],[Order_No],[Item_Code],[Notes],[QTY] ,[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID]) VALUES ('OR_Dt_006','Ord_003','Item003','Very Spicy',4 ,GETDATE(),'SHANU' ,GETDATE(),'SHANU')2、簡單的Select查詢語句
Select查詢語句是SQL中最基本也是最重要的DML語句之一。那么什么是DML？DML全稱Data Manipulation Language（數據操縱語言命令），它可以使用戶能夠查詢數據庫以及操作已有數據庫中的數據。
下面我們在SQL Server中用select語句來查詢我的姓名(Name)：
SELECT 'My Name Is SYED SHANU'-- With Column Name using 'AS'SELECT 'My Name Is SYED SHANU' as 'MY NAME'-- With more then the one Column SELECT 'My Name' as 'Column1', 'Is' as 'Column2', 'SYED SHANU' as 'Column3'
在數據表中使用select查詢：
-- To Display all the columns from the table we use * operator in select Statement.Select * from ItemMasters-- If we need to select only few fields from a table we can use the Column Name in Select Statement.Select Item_Code ,Item_name as Item ,Price ,Description ,In_DATE FROM ItemMasters
3、合計和標量函數
合計函數和標量函數都是SQL Server的內置函數，我們可以在select查詢語句中使用它們，比如Count(), Max(), Sum(), Upper(), lower(), Round()等等。下面我們用SQL代碼來解釋這些函數的用法：
select * from ItemMasters-- Aggregate-- COUNT() -> returns the Total no of records from table , AVG() returns the Average Value from Colum,MAX() Returns MaX Value from Column-- ,MIN() returns Min Value from Column,SUM() sum of total from ColumnSelect Count(*) TotalRows,AVG(Price) AVGPrice ,MAX(Price) MAXPrice,MIN(Price) MinPrice,Sum(price) PriceTotal FROM ItemMasters-- Scalar -- UCASE() -> Convert to Upper Case ,LCASE() -> Convert to Lower Case,-- SUBSTRING() ->Display selected char from column ->SUBSTRING(ColumnName,StartIndex,LenthofChartoDisplay)--,LEN() -> lenth of column date,-- ROUND() -> Which will round the valueSELECT UPPER(Item_NAME) Uppers,LOWER(Item_NAME) Lowers, SUBSTRING(Item_NAME,2,3) MidValue,LEN(Item_NAME) Lenths ,SUBSTRING(Item_NAME,2,LEN(Item_NAME)) MidValuewithLenFunction, ROUND(Price,0) as Rounded FROM ItemMasters
4、日期函數
在我們的項目數據表中基本都會使用到日期列，因此日期函數在項目中扮演著非常重要的角色。有時候我們對日期函數要非常的小心，它隨時可以給你帶來巨大的麻煩。在項目中，我們要選擇合適的日期函數和日期格式，下面是一些SQL日期函數的例子：
-- GETDATE() -> to Display the Current Date and Time-- Format() -> used to display our date in our requested formatSelect GETDATE() CurrentDateTime, FORMAT(GETDATE(),'yyyy-MM-dd') AS DateFormats, FORMAT(GETDATE(),'HH-mm-ss')TimeFormats, CONVERT(VARCHAR(10),GETDATE(),10) Converts1, CONVERT(VARCHAR(24),GETDATE(),113), CONVERT(NVARCHAR, getdate(), 106) Converts2 ,-- here we used Convert Function REPLACE(convert(NVARCHAR, getdate(), 106), ' ', '/') Formats-- Here we used replace and --convert functions. --first we convert the date to nvarchar and then we replace the '' with '/' select * from ItemmastersSelect ITEM_NAME,IN_DATE CurrentDateTime, FORMAT(IN_DATE,'yyyy-MM-dd') AS DateFormats, FORMAT(IN_DATE,'HH-mm-ss')TimeFormats, CONVERT(VARCHAR(10),IN_DATE,10) Converts1, CONVERT(VARCHAR(24),IN_DATE,113), convert(NVARCHAR, IN_DATE, 106) Converts2 ,-- here we used Convert Function REPLACE(convert(NVARCHAR,IN_DATE, 106), ' ', '/') Formats FROM Itemmasters
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