mta database fundamentals

a query statement that is treated as a result set is called a subquery, as with a normal table. SELECT * FROM (SELECT * from Student where age>20) As name Select Rom_number () over (order by age DESC) as Romnum Name,age,chinese,math from Student This stage of the database read, and I also mastered a lot of database query, build table, update, delete and other statements, the following should be the stud

Connection query: You can implement multiple table queries by using the Join operator. Connection is the main feature of relational database model, and it is also a sign that distinguishes it from other types of database management system.Common two link operators:1.join on2.union in the relational database management system, the relationship between the data

Database design is divided into requirements analysis, conceptual design, logical design and database physical design four parts.1. Demand Analysis:Stage Result: Requirement specification.2. Conceptual structure Design:Conceptual structure design is to synthesize, generalize and abstract the user's needs, and form a conceptual model independent of the specific DBMS.Conceptual structure design typically has

so will cause the engine to discard full table scans using the index, such as: Select ID from t where num=10 or num=20 You can query this:Select ID from t where num=10UNION ALLSelect ID from t where num=20 (The difference between Union and union all is that union automatically compresses duplicate results in multiple result sets, and union ALL displays all results, whether duplicates or not.) ) 5. If there is a stored procedure Proc1 (employee number, month) in MySQL to query the employee's sa

duplicates:0 warnings:0 2) Delete field mysql> ALTER TABLE student drop column sgender;Query OK, 0 rows affected (0.03 sec)records:0 duplicates:0 warnings:0 3) Modify the field type mysql> ALTER TABLE student Modify column remark varchar (100);Query OK, 0 rows affected (0.07 sec)records:0 duplicates:0 warnings:0 4) Modify the field name mysql> ALTER TABLE student change column sgender gender varchar (2)

convert.tostring () (String) From the results can be seen: converted to int , the rounding calculation, this and the display type conversion is different, before we use the display type conversion to convert 58.5 to int is the result of The value after the decimal point is discarded directly. Summary: The range of methods used for various conversionsImplicit type conversions: often used between numeric types, converting a numeric type with a small