非常惭愧用了这么久的mysql居然没有用过outer join和inner join,对outer join的认识也仅是知道它是外连结,至于什么用途都不清楚,至于还有没有left outer join更是不得而知,某天有人问起,才想起自己mysql知识的贫乏,赶紧找了一下网上的left join,right join,inner join,outer join的用法来学习一下 下面的内容转载...
TheRIGHT OUTER JOINworks exactly the opposite of theLEFT OUTER JOIN. In this operation, all the rows from the right table are returned, along with the rows from the left table that match the ones in the right table. Missing values in the left table are given a value ofNULL. Example: C...
A=T1.A LEFT JOIN T3 ON T3.B=T1.B WHERE T3.C > 0 If the WHERE condition is null-rejected for an outer join operation in a query, the outer join operation is replaced by an inner join operation. For example, in the preceding query, the second outer join is null-rejected and ...
有些数据库的语法会是LEFT OUTER JOIN。 LEFT JOIN 查询实例 (Example) 现在我们想查询所有客户与其订单状况的数据,我们可以作一个 LEFT JOIN 查询: SELECT , orders.Order_No FROM customers LEFT JOIN orders ON customers.C_Id=orders.C_Id; 查询结果如下: LEFT JOIN会返回左侧数据表中所有数据列,就算没有...
左外连接(left outer join): 以第一个关系(左表)为主,在第二个关系(右表)中根据匹配条件找到满足条件的元素,并把它们连接起来,如果右表中没有对应的元素,则在相应位置上的值为NULL,左外连接的结果行数等于左表的行数 右外连接(right outer join):以第二个关系(右表)为主,在第一个关系(左表)中根据匹...
Simple Example of the MySQL FULL JOIN/FULL OUTER JOIN Let us now make a full join on the Students table and Marks table such that we see all the records from both tables. The first step is to find the commonly related columns between the two tables. In this case, they are ID from ...
Right joins are converted to equivalent left joins, as described in Section 10.2.1.10, “Outer Join Simplification”. For a LEFT JOIN, if the WHERE condition is always false for the generated NULL row, the LEFT JOIN is changed to an inner join. For example, the WHERE clause would be ...
左外连接(left outer join): 如果数据不存在,左表记录会出现,而右表以null 填充 右外连接(right outer join):如果数据不存在,右表记录会出现,而左表以null 填充 七、其他常用语句 1、CREATE INDEX create index 用于在一个或多个列上创建索引 CREATE INDEX indexname ON tablename(column [ASC | DESC],...
修改SQL 语法配置规则支持 FULL JOIN 语法。运用从 BISON 学到知识,修改sql/sql_yacc.yy如下,+ 号表示新增内容,- 号表示原版内容。 在sql/parser_yystype.h的enum PT_joined_table_type中添加JTT_FULL、JTT_NATURAL_FULL枚举类型 因为TABLE的结构体中的数据成员 outer_join 只代表是否是 left join 或者 outer ...
table_reference[INNER | CROSS] JOINtable_factor [join_condition] 3. MySQL中的外连接,分为左外连接和右连接, 即除了返回符合连接条件的结果之外,还要返回左表(左连接)或者右表(右连接)中不符合连接条件的结果,相对应的使用NULL对应。 a. LEFT [OUTER] JOIN ...