three-valued-logic | 易学教程

Options for eliminating NULLable columns from a DB model (in order to avoid SQL's three-valued logic)?

阅读更多关于 Options for eliminating NULLable columns from a DB model (in order to avoid SQL's three-valued logic)?

问题 Some while ago, I've been reading through the book SQL and Relational Theory by C. J. Date. The author is well-known for criticising SQL's three-valued logic (3VL). 1) The author makes some strong points about why 3VL should be avoided in SQL, however he doesn't outline how a database model would look like if nullable columns weren't allowed . I've thought on this for a bit and have come up with the following solutions. If I missed other design options, I would like to hear about them! 1)

NOT IN does not produce same results as NOT EXISTS

阅读更多关于 NOT IN does not produce same results as NOT EXISTS

问题 These are rather basic statements. I have a list of graphics which are linked to items in another table. I want to check how many of the graphics are not in use and can theoretically be deleted. So first I used the NOT IN clause: SELECT [GraphicNr] ,[Graphicfile] FROM [dbo].[Graphic] WHERE graphicnr NOT IN (SELECT graphicnr FROM dbo.Komp) Which gave zero results, which seemed weird to me. After rewriting it to a NOT EXISTS, I got about 600 results: SELECT [GraphicNr] ,[Graphicfile] FROM [dbo]

NOT IN does not produce same results as NOT EXISTS

阅读更多关于 NOT IN does not produce same results as NOT EXISTS

These are rather basic statements. I have a list of graphics which are linked to items in another table. I want to check how many of the graphics are not in use and can theoretically be deleted. So first I used the NOT IN clause: SELECT [GraphicNr] ,[Graphicfile] FROM [dbo].[Graphic] WHERE graphicnr NOT IN (SELECT graphicnr FROM dbo.Komp) Which gave zero results, which seemed weird to me. After rewriting it to a NOT EXISTS, I got about 600 results: SELECT [GraphicNr] ,[Graphicfile] FROM [dbo].[Graphic] a WHERE NOT EXISTS (SELECT graphicnr FROM dbo.komp b WHERE a.GraphicNr = b.GraphicNr) So I

Example of three valued logic in SQL Server

阅读更多关于 Example of three valued logic in SQL Server

问题 I understand that SQL uses three valued logic but I am having trouble understanding how to use this in practice, especially why TRUE || NULL = True and FALSE && NULL = False instead of evaluating to null . Here are the three valued truth tables that apply to SQL Server: I found a couple explanations of three valued logic online but I cannot find any real code examples of this in use. Can someone show me a code example using three valued logic to help me understand this a little better? 回答1: