【SQL进阶教程】HAVING子句的力量
表的记录是没有顺序的，而且SQL也没有排序的运算符。SQL会将多条记录作为一个集合来处理。
寻找缺失的编号假设现有一张带有“连续编号”列的表，如表SeqTbl所示。查询这张表里是否存在数据丢失。
如果将表整体看作一个集合，就可以像下面这样解决这个问题。
123SELECT '存在缺失的编号' AS
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[LeetCode] Problem 462 - Minimum Moves to Equal Array Elements II
Given a non-empty integer array, find the minimum number of moves required to make all array elements equal, where a move is incrementing a selected e
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[LeetCode] Problem 453 - Minimum Moves to Equal Array Elements
Given a non-empty integer array of size n, find the minimum number of moves required to make all array elements equal, where a move is incrementing n
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[LeetCode] Problem 447 - Number of Boomerangs
Given n points in the plane that are all pairwise distinct, a “boomerang” is a tuple of points (i, j, k) such that the distance between i and j equals
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[LeetCode] Problem 443 - String Compression
Given an array of characters, compress it in-place.
The length after compression must always be smaller than or equal to the original array.
Every ele
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【SQL进阶教程】三值逻辑和NULL
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NULL既不是值也不是变量。NULL只是一个表示“没有值”的标记。对NULL使用比较谓词后得到的结果总是unknown。真值unknown是因关系数据库采用了NULL而被引入的的“第三个真值”，是明确的布尔型的真值。
要想解决NULL带来的各种问题，最佳方法应该是往表里添加NOT NULL约束来尽力
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The English We Speak 2
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To downsizeto describe cutting a number of jobs and making employees redundant.some people have started applying the term to their own situations.
Ex
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[LintCode] Problem 648 - Unique Word Abbreviation
An abbreviation of a word follows the form<first letter><number><last letter>. Below are some examples of word abbreviations:
123456
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[LintCode] Problem 779 - Generalized Abbreviation
Write a function to generate the generalized abbreviations of a word.(order does not matter)
ExampleNo.1Input:word = “word”,
Output:[“word”, “1ord”, “
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【SQL进阶教程】自连接的用法
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与多表之间进行的普通连接相比，自连接的性能开销更大（特别是与非等值连接结合使用的时候），因此用于自连接的列推荐使用主键或者在相关列上建立索引。
可重排列、排列、组合假设有一张存放了商品名称及价格的表，表里有“苹果、橘子、香蕉”这3条记录。在生成用于查询销售额的报表等的时候，有时会需要获取这些商品的组
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