By Bill Karwin
Invoice Karwin has helped hundreds of thousands of individuals write larger SQL and construct better relational databases. Now he's sharing his choice of antipatterns the commonest blunders he's pointed out in these hundreds of thousands of requests for help.
Most builders aren't SQL specialists, and many of the SQL that will get used is inefficient, not easy to take care of, and occasionally simply undeniable incorrect. This booklet indicates you all of the universal errors, after which leads you thru the easiest fixes. What's extra, it exhibits you what's at the back of those fixes, so you'll examine much approximately relational databases alongside the best way.
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Extra resources for SQL Antipatterns: Avoiding the Pitfalls of Database Programming
Sorting through millions of comments every time someone views the website is impractical. There must be a better way to store the threads of comments so you can retrieve a whole discussion thread simply and efficiently. 1 Objective: Store and Query Hierarchies It’s common for data to have recursive relationships. Data may be organized in a treelike or hierarchical way. In a tree data structure, each entry is called a node. A node may have a number of children and one parent. The top node, which has no parent, is called the root.
I’ve heard some software developers claim that their table doesn’t need a primary key. Sometimes these programmers want to avoid the imagined overhead of maintaining a unique index, or else they have tables with no columns they can use for this purpose. A primary key constraint is important when you need to do the following: • Prevent a table from containing duplicate rows • Reference individual rows in queries • Support foreign key references If you don’t use primary key constraints, you create a chore for yourself: checking for duplicate rows.
2) Ollie: I think it’s a null pointer. (3) Fran: No, I checked for that. (5) Ollie: Yes, that’s a bug. (6) Fran: Yes, please add a check. (7) Kukla: That fixed it. 4: Closure Table illustration The queries to retrieve ancestors and descendants from this table are even more straightforward than those in the Nested Sets solution. descendant = 6; To insert a new leaf node, for instance a new child of comment #5, first insert the self-referencing row. sql DELETE FROM TreePaths WHERE descendant IN (SELECT descendant FROM TreePaths WHERE ancestor = 4); Notice that if you delete rows in TreePaths, this doesn’t delete the comments themselves.