Introduction to SQL

• SQL是一种结构化查询语言
• SQL语言包括DQL（数据查询）、DCL（数据控制）、DDL（数据定义）、DML（数据操纵）
  • 因为关系模式中，实体与实体之间用关系表示联系，所以操作符比较简单，每种操作只需要一种操作符表示
• SQL语言的操作对象都是集合（查询、插入、修改、删除）
• SQL语言既是自含式语言，同时也是嵌入式语言
• 语言简洁
  • 核心功能只有九个

Select-From-Where Statements

SELECT desired attributes FROM one or more tables WHERE condition about tuples of the tables

Meaning of Single-Relation Query

• Begin with the relation in the FROM clause.
• Apply the selection indicated by the WHERE clause.
• Apply the extended projection indicated by the SELECT clause

FROM确定数据源，WHERE进行数据的筛选，SELETE对属性进行投影

Operational Semantics

• Think of a tuple variable visiting each tuple of the relation mentioned in FROM.
• Check if the “current” tuple satisfies the WHERE clause.
• If so, compute the attributes or expressions of the SELECT clause using the components of this tuple.

可以认为元组变量将访问所有FROM中涉及的关系，然后循环的检验是否满足WHERE中的条件，最后将满足条件的数据放到返回结果中，并且进行扩展投影

* In SELECT clauses

When there is one relation in the FROM clause, * in the SELECT clause stands for “all attributes of this relation.”

SELETE中既可以是属性也可以是常量

Complex Conditions in WHERE Clause

PATTERNS

• A condition can compare a string to a pattern by: <Attribute> LIKE <pattern> or <Attribute> NOT LIKE <pattern>
• Pattern is a quoted string with % = “any string”; _ = “any character. ”
• % 0~任意多个字符
• _ 任意单个字符
• [] 集合范围内的任意单个字符
• [^] 不在集合范围内的任意单个字符
• […-…] 前一字符至后一字符中的任一字符
• ESCAPE 取消后面通配字符的通配作用（也就是说有时候需要使用到字符本身）

NULL Values

• Tuples in SQL relations can have NULL as a value for one or more components.（SQL关系中允许元组有一种或多种属性是NULL）
• Meaning depends on context. Two common cases:

Missing value : e.g., we know Joe’s Bar has some address, but we don’t know what it is.（缺省值） Inapplicable : e.g., the value of attribute spouse for an unmarried person.（不适用值）

Three-Valued Logic

• To understand how AND, OR, and NOT work in 3-valued logic, think of TRUE = 1, FALSE = 0, and UNKNOWN = ½.
• AND = MIN; OR = MAX, NOT(x) = 1-x.
• Example:

TRUE AND (FALSE OR NOT(UNKNOWN)) = MIN(1, MAX(0, (1 - ½ ))) = MIN(1, MAX(0, ½ )) = MIN(1, ½ ) = ½.

Multirelation Queries

• Interesting queries often combine data from more than one relation.
• We can address several relations in one query by listing them all in the FROM clause.
• Distinguish attributes of the same name by “<relation>.<attribute>” .

Example: Joining Two Relations

Using relations Likes(drinker, beer) and Frequents(drinker, bar), find the beers liked by at least one person who frequents Joe’s Bar.

SELECT beer
ROM Likes, Frequents
WHERE bar = ’Joe’’s Bar’ AND
Frequents.drinker = Likes.drinker;

Aggregations

• SUM, AVG, COUNT, MIN, and MAX can be applied to a column in a SELECT clause to produce that aggregation on the column.
• Also, COUNT(*) counts the number of tuples.

NULL’s Ignored in Aggregation

• NULL never contributes to a sum, average, or count, and can never be the minimum or maximum of a column.
• But if there are no non-NULL values in a column, then the result of the aggregation is NULL.

NULL值不参与聚合函数的运算

当表为空集时，返回结果为0

Grouping

• We may follow a SELECT-FROM-WHERE expression by GROUP BY and a list of attributes.（在select-from-where语句后面接GroupBy和一系列属性）
• The relation that results from the SELECT-FROM-WHERE is grouped according to the values of all those attributes, and any aggregation is
• applied only within each group.（select返回的结果将会被Group语句进行分组，并且groupby只在分组内应用）

select语句后面是可以接单属性的，当且仅当group后面接的属性

HAVING Clauses

• HAVING <condition> may follow a GROUP BY clause.
• If so, the condition applies to each group, and groups not satisfying the condition are eliminated.

注意：

• WHERE：在分组前过滤行。
• GROUP BY：对过滤后的行分组。
• HAVING：对分组后的聚合结果过滤。

例题：