sql查詢語句的處理步驟如下:
--查詢組合字段
(5)select (5-2) distinct(5-3) top(<top_specification>)(5-1)<select_list>
--連表
(1)from (1-J)<left_table><join_type> join <right_table> on <on_predicate>
(1-A)<left_table><apply_type> apply <right_table_expression> as <alias>
(1-P)<left_table> pivot (<pivot_specification>) as <alias>
(1-U)<left_table> unpivot (<unpivot_specification>) as <alias>
--查詢條件
(2)where <where_pridicate>
--分組
(3)group by <group_by_specification>
--分組條件
(4)having<having_predicate>
--排序
(6)order by<order_by_list>
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說明:
1、順序爲有1-6,6個大步驟,然後細分,5-1,5-2,5-3,由小變大順序,1-J,1-A,1-P,1-U,爲並行次序。如果不夠明白,接下來我在來個流程圖看看。
2、執行過程中也會相應的產生多個虛擬表(下面會有提到),以配合最終的正確查詢。
sql查詢語句處理步驟流程圖
準備實例,創建表,插入數據,寫要分析的實例查詢語句
1.首先創建兩個表
2.創建兩個表,並插入表數據,腳本如下
USE [test]
GO
/****** Object: Table [dbo].[Member] Script Date: 2014/12/22 14:05:17 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[Member](
[id] [int] IDENTITY(1,1) NOT NULL,
[Name] [nvarchar](30) NULL,
[phone] [varchar](15) NULL,
CONSTRAINT [PK_MEMBER] PRIMARY KEY CLUSTERED
(
[id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
/****** Object: Table [dbo].[Order] Script Date: 2014/12/22 14:05:17 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[Order](
[id] [int] IDENTITY(1,1) NOT NULL,
[member_id] [int] NULL,
[status] [int] NULL,
[createTime] [datetime] NULL,
CONSTRAINT [PK_ORDER] PRIMARY KEY CLUSTERED
(
[id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET IDENTITY_INSERT [dbo].[Member] ON
GO
INSERT [dbo].[Member] ([id], [Name], [phone]) VALUES (1, N'張龍豪', N'18501733702')
GO
INSERT [dbo].[Member] ([id], [Name], [phone]) VALUES (2, N'Jim', N'15039512688')
GO
INSERT [dbo].[Member] ([id], [Name], [phone]) VALUES (3, N'Tom', N'15139512854')
GO
INSERT [dbo].[Member] ([id], [Name], [phone]) VALUES (4, N'Lulu', N'15687425583')
GO
INSERT [dbo].[Member] ([id], [Name], [phone]) VALUES (5, N'Jick', N'13528567445')
GO
SET IDENTITY_INSERT [dbo].[Member] OFF
GO
SET IDENTITY_INSERT [dbo].[Order] ON
GO
INSERT [dbo].[Order] ([id], [member_id], [status], [createTime]) VALUES (1, 1, 3, CAST(0x0000A40900B3BBFB AS DateTime))
GO
INSERT [dbo].[Order] ([id], [member_id], [status], [createTime]) VALUES (2, 2, 1, CAST(0x0000A40900B3CEF2 AS DateTime))
GO
INSERT [dbo].[Order] ([id], [member_id], [status], [createTime]) VALUES (3, 3, 4, CAST(0x0000A40900B3D2D0 AS DateTime))
GO
INSERT [dbo].[Order] ([id], [member_id], [status], [createTime]) VALUES (4, 4, 0, CAST(0x0000A40900B3D660 AS DateTime))
GO
INSERT [dbo].[Order] ([id], [member_id], [status], [createTime]) VALUES (5, 5, 1, CAST(0x0000A40900B3D9B9 AS DateTime))
GO
INSERT [dbo].[Order] ([id], [member_id], [status], [createTime]) VALUES (6, 6, 2, CAST(0x0000A40900B3DFEA AS DateTime))
GO
INSERT [dbo].[Order] ([id], [member_id], [status], [createTime]) VALUES (7, NULL, 0, CAST(0x0000A40900E34971 AS DateTime))
GO
SET IDENTITY_INSERT [dbo].[Order] OFF
GO
ALTER TABLE [dbo].[Order] ADD DEFAULT (getdate()) FOR [createTime]
GO
EXEC sys.sp_addextendedproperty @name=N'MS_Description', @value=N'編號' , @level0type=N'SCHEMA',@level0name=N'dbo', @level1type=N'TABLE',@level1name=N'Member', @level2type=N'COLUMN',@level2name=N'id'
GO
EXEC sys.sp_addextendedproperty @name=N'MS_Description', @value=N'姓名' , @level0type=N'SCHEMA',@level0name=N'dbo', @level1type=N'TABLE',@level1name=N'Member', @level2type=N'COLUMN',@level2name=N'Name'
GO
EXEC sys.sp_addextendedproperty @name=N'MS_Description', @value=N'電話' , @level0type=N'SCHEMA',@level0name=N'dbo', @level1type=N'TABLE',@level1name=N'Member', @level2type=N'COLUMN',@level2name=N'phone'
GO
EXEC sys.sp_addextendedproperty @name=N'MS_Description', @value=N'會員表' , @level0type=N'SCHEMA',@level0name=N'dbo', @level1type=N'TABLE',@level1name=N'Member'
GO
EXEC sys.sp_addextendedproperty @name=N'MS_Description', @value=N'編號' , @level0type=N'SCHEMA',@level0name=N'dbo', @level1type=N'TABLE',@level1name=N'Order', @level2type=N'COLUMN',@level2name=N'id'
GO
EXEC sys.sp_addextendedproperty @name=N'MS_Description', @value=N'會員編號' , @level0type=N'SCHEMA',@level0name=N'dbo', @level1type=N'TABLE',@level1name=N'Order', @level2type=N'COLUMN',@level2name=N'member_id'
GO
EXEC sys.sp_addextendedproperty @name=N'MS_Description', @value=N'訂單狀態' , @level0type=N'SCHEMA',@level0name=N'dbo', @level1type=N'TABLE',@level1name=N'Order', @level2type=N'COLUMN',@level2name=N'status'
GO
EXEC sys.sp_addextendedproperty @name=N'MS_Description', @value=N'下單日期' , @level0type=N'SCHEMA',@level0name=N'dbo', @level1type=N'TABLE',@level1name=N'Order', @level2type=N'COLUMN',@level2name=N'createTime'
GO
EXEC sys.sp_addextendedproperty @name=N'MS_Description', @value=N'訂單表' , @level0type=N'SCHEMA',@level0name=N'dbo', @level1type=N'TABLE',@level1name=N'Order'
GO
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3.編寫咱們要解析的查詢語句,即本篇要查詢的實例語句。
select top(4) status , max(m.id) as maxMemberID
from [dbo].[Member] as m right outer join [dbo].[Order] as o
on m.id=o.member_id
where m.id>0
group by status
having status>=0
order by maxMemberID asc
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實例語句分步驟分析
1.從from開始
1.1 加載左表
from [dbo].[Member] as m
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查詢結果:member表中的所有數據
1.2 這裏應該是 right outer join ,但是這裏在sql中被定義分解爲2個步驟,即join ,right outer join 。表達式關鍵字從左到右,依次執行。
join [dbo].[Order] as o
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查詢結果:存入虛擬表vt1,爲兩個表的笛卡爾集合。這裏你或許不明白什麼叫笛卡爾集合,我打個比方給說說,還望不要嫌棄,就是小朋友握手問題,A班裏有3個學生(看作一個表的三條數據),B班裏有2個學生(看作另外一個表的2條數據).B班小朋友跟A班小朋友搞聯歡晚會,首先要每個人都要確保跟另外一個班的同學我一下手,那麼交叉出來的集合就是(2*3=6)有6條不同的軌跡。這個軌跡的集合就是笛卡爾集合。如果你還不明白,我再說下,就是m(5條數據)表中的第一條數據跟o(7條數據)表中的所有數據握下手,有7條,然後依次類推共有35條不同的數據。這裏的null值也是要加進來的。
1.3、on 篩選器
on m.id=o.member_id
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查詢結果如下:
從上一步的笛卡爾集中的35條數據中刪除掉不匹配的行,得到5條數據,存入虛擬表Vt2。
1.4 、添加外部行(outer row)
right outer join [dbo].[Order] as o
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查詢結果如下:
右表(order)作爲保留表,把剩餘的數據重新添加到上一步的虛擬表vt2中,生成虛擬表vt3。
2. where 階段
where m.id>0
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查詢結果:存入虛擬表vt4,爲篩選的條件爲true的結果集,這裏加入一個記憶點,就是,where的篩選刪除爲永久的,而on的篩選刪除爲暫時的,因爲on篩選過後,有可能會經過outer添加外部行,重新把數據加載回來,而where則不能。
3.group by分組
group by status
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查詢結果:存入vt5,以status列的數值開始分組,即status列,值一樣的分爲一組,這裏的兩個null在三值邏輯中被視爲true。三值邏輯:true,false,null。此三值,null爲未知,是數據的邏輯特色,有的地方兩個null相等爲ture,在有些地方則爲false。這個你百度下看看有很多講解。
4.having 篩選
having status>=0
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查詢結果:篩選分好組的組數據,把不滿足條件的刪除掉
5.select 查詢挑揀計算列
5.1、計算表達式
select status , max(m.id)
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查詢結果:從分過組的數據中計算各個組中的最大m.id,列出要篩選顯示的列。
5.2、distinct過濾重複
5.3、top 結合order by 篩選 多少行,但這裏的數據沒有排序只是把多少行數據列出來而已。
6.order by
排序顯示
至此,一個完整的sql查詢執行完畢。
希望能對大家有所幫助