三、结论 掌握CASE 语句使我们能够直接在 SQL 查询中执行复杂的条件逻辑。无论对数据进行分类、对结果进行优先级排序,还是进行过滤,CASE 语句都是一个宝贵工具。通过了解其语法并探索实际示例,可以在数据库操作中充分利用 SQL CASE 语句的潜力。
importpsycopg2importpandasaspdfromsqlalchemyimportcreate_engine,text# 连接数据库取数engine=create_engine('postgresql+psycopg2://postgres:123456@127.0.0.1:5432/sql_advanced')df=pd.read_sql_query('SELECT * FROM poptbl',engine)# 创建一个字典,用来映射pref_name和districtdistrict_map={'德岛':'九州','...
read_sql_query('SELECT * FROM PopTbl2', engine) PopTbl2['sex'] = PopTbl2['sex'].map({'1': '男', '2': '女'}) savecol = ['德岛', '香川', '爱媛', '高知'] #按tsex列进行分组,并计算总人口 grouped = PopTbl2.groupby('sex') nationwide = grouped.agg({'population': 'sum...
您可以使用 AS 子句创建新的列名。...SQL SELECT INTO 示例以下 SQL 语句创建 Customers 的备份副本: SELECT * INTO CustomersBackup2017 FROM Customers; 以下...只需添加一个导致查询不返回数据的 WHERE 子句: SELECT * INTO newtable FROM oldtable WHERE 1 = 0; SQL INSERT INTO SELECT 语句...Country...
Azure SQL 托管实例 适用于:Azure SQL 数据库 和 SQL Server(从 SQL Server 2017 (14.x) 开始) 本机编译的 T-SQL 模块支持 CASE 表达式。 下面的示例演示了在查询中使用 CASE 表达式的方法。 -- Query using a CASE expression in a natively compiled stored procedure...
root@mysqldb 13:03: [test]> create table T1(id int); Query OK, 0 rows affected (0.08 sec) root@mysqldb 13:03: [test]> show tables; +---+ | Tables_in_test | +---+ | T1 | | t1 | | t2 | +---+ 3 rows in set (0.00 sec) root@mysqldb 13:03: [test]> select * from...
possible. Aggregate expressions that appear in WHEN arguments to aCASEexpression are evaluated first, then provided to theCASEexpression. For example, the following query produces a divide by zero error when producing the value of the MAX aggregate. This occurs prior to evaluating theCASEexpression....
possible. Aggregate expressions that appear in WHEN arguments to aCASEexpression are evaluated first, then provided to theCASEexpression. For example, the following query produces a divide by zero error when producing the value of the MAX aggregate. This occurs prior to evaluating theCASEexpression....
背景:性能应该是功能的一个重要参考,特别是在大数据的背景之下!写SQL语句时如果仅考虑业务逻辑,而不去考虑语句效率问题,有可能导致严重的效率问题,导致功能不可用或者资源消耗过大。其中的一种情况是,处理每日增量数据的程序,实际执行过程中可能会进行全表扫描,效
possible. Aggregate expressions that appear in WHEN arguments to aCASEexpression are evaluated first, then provided to theCASEexpression. For example, the following query produces a divide by zero error when producing the value of the MAX aggregate. This occurs prior to evaluating theCASEexpression....