Oracle Development <<sql Cookbook>> Learning Notes Collation: First chapter Search Records

1. Write the program to list the required columns than direct write select * More intuitive. In this way, the performance of row data retrieval is the same, and the performance of column data retrieval is improved; When you select the required columns, you reduce IO and network transfer time to improve performance.

The 2.WHERE clause filters records with predicates such as =,<,>,<=,>=,!, <> and And,or.

The 3.NULL value is judged by IS null and is not NULL.

4. A column alias is especially important in a subquery.

5. Use | | To stitch the values of the columns.

6. Use the case when end clause to implement IF-ELSE judgment.

7. When top n is taken, it is important to note that rownum can fetch only the first n rows, not the nth row, and the nth row using Row_number ().

8. Random data collection (useful when making data):

SELECT *  from (    SELECT  ename, Job          from  emp          ORDERby  Dbms_random.value ())  WHERE<=5

9. Use the NVL or coalesce or case when end clauses to handle null values, as shown in the following example:

SELECT empno,       ename,        job,         Comm Comm_raw,       coalesce(comm,0-- or NVL (comm,0) Commfrom   emp;        

10. Use a like fuzzy match:% pass with _ placeholder.

Oracle Development <<sql Cookbook>> Learning Notes Collation: First chapter Search Records