例如 SELECT * FROM users RIGHT JOIN orders ON users.user_id = orders.user_id; ，会返回所有订单，以及下单用户的信息，如果某个订单没有对应的用户（理论上很少见，但在数据不完整时可能出现），则用户相关列显示为NULL 。3. 子查询子查询是指在一个查询中嵌套另一个查询。作为条件的子查询
SELECT * FROM (VALUES (1,2) , (3,4) ) t1 (c1, c2) The only other standard-conforming alternative is to use a dummy table in the from clause. Databases that do not allow select without from usually ship with tables for this purpose (e.g., DUAL in the Oracle database or SYSIBM....
ORDER BY 子句可以基于查询中使用的任何表中的任何字段来进行排序,无论该列是否包含在SELECT列表中。 5. 使用GROUP BY 子句聚集数据 一旦在查询语句中使用了GROUP BY,SELECT列表中的每一列要么包含在GROUP BY列表中,要不包含在聚集中。 当聚集不与GROUP BY一起使用时,聚集只能与其他聚集一起位于SELECT列表中,而...
select * from a left join b on a.id = b.id where a.id = 0; 这个SQL是联合多张表进行联合查询,还有其他查询条件,执行得出结果时间为1432ms,超过1秒则为慢 SQL,这里联合两张表,如果相关的数据量较大,则执行速度会较慢。我们可以将 SQL 拆分为两个语句执行,拆为以下两个 SQL 分步执行: select * f...
1)LEFT OUTER JOIN,简称LEFT JOIN,左外连接(左连接) 结果集保留左表的所有行,但只包含第二个表与第一表匹配的行。第二个表相应的空行被放入NULL值。 依然沿用内链接的例子 (1)使用左连接查询学生的信息,其中包括学生ID,学生姓名和专业名称。 SELECTStudents.ID,Students.Name,Majors.NameASMajorNameFROMStudents...
left semi join语句的作用和 in/exists 作用是一样的,具体语法: SELECT A.* FROM A where id in (select id from B) SELECT A.* FROM A left semi join B ON A.id=B.id 注意事项: left semi join 的限制是:join 子句中右边的表只能在 on 子句中设置过滤条件,在 where 子句、select 子句或其他地...
SELECTRTRIM(p.FirstName) +' '+LTRIM(p.LastName)ASName, d.CityFROMPerson.PersonASpINNERJOINHumanResources.Employee eONp.BusinessEntityID = e.BusinessEntityIDINNERJOIN(SELECTbea.BusinessEntityID, a.CityFROMPerson.AddressASaINNERJOINPerson.BusinessEntityAddressASbeaONa.AddressID = bea.AddressID )ASdO...
-- 从table_1中选择a这一列selectafromtable_1 2. 表连接 -- table_1中有id,age; table_2中有id,sex。想取出id,age,sex 三列信息-- 将table_1,table_2 根据主键id连接起来selecta.id,a.age,b.sexfrom(selectid,agefromtable_1) a--将select之后的内容存为临时表ajoin(selectid, sexfromtable_2...
TheSELECT ... FROM <table>statement has a new clauseFOR SYSTEM_TIME, with five temporal-specific subclauses to query data across the current and history tables. This newSELECTstatement syntax is supported directly on a single table, propagated through multiple joins, and through views on top ...
If you specify a join hint in the same query'sFROMclause for a specific table pair, this join hint takes precedence in the joining of the two tables. The query hints, though, must still be honored. The join hint for the pair of tables might only restrict the selection of allowed join...