加入:
SQL 中的 JOIN 用于根据多个表之间的匹配条件组合来自多个表的数据。使用 JOIN 语句将数据组合到新列中。
考虑两个表:
男孩们
女孩们
例子:
sql>
SELECT Boys.Name, Boys.Age, Girls.Address,
FROM Boys
INNER JOIN Girls
ON Boys.Rollno = Girls.Rollno;
结果表是:
联合:
SQL 中的 UNION 用于组合两个或多个 SELECT 语句的结果集。使用 UNION 语句组合的数据将结果转化为新的不同行。
例子:
sql>
SELECT Name
FROM Boys
WHERE Rollno < 16
UNION
SELECT Name
FROM Girls
WHERE Rollno > 9
结果表是:
SQL 中 JOIN 和 UNION 的区别:
JOIN | UNION |
---|---|
JOIN combines data from many tables based on a matched condition between them. | SQL combines the result-set of two or more SELECT statements. |
It combines data into new columns. | It combines data into new rows |
Number of columns selected from each table may not be same. | Number of columns selected from each table should be same. |
Datatypes of corresponding columns selected from each table can be different. | Datatypes of corresponding columns selected from each table should be same. |
It may not return distinct columns. | It returns distinct rows. |