Restricting and Sorting Data
Objectives
After completing this lesson,you should be able to do the following:
- Limit the rows that are retrieved by a query
- Sort the rows that are retrieved by a query
Character Strings and Dates
列别名应当使用双引号
字符串拼接或者日期信息,以及where子句中的数据,需要使用单引号
- Character strings and date values are enclosed by single quotation marks
- Character values are case-sensitive,and date values are format-sensitive.
- The default date format is DD-MON-RR.
SQL> SELECT last_name,job_id,department_id 2 FROM employees 3 WHERE last_name = 'Grant'; LAST_NAME JOB_ID DEPARTMENT_ID ------------------------- ---------- ------------- Grant SH_CLERK 50 Grant SA_REP SQL> SELECT last_name,job_id,department_id 2 FROM employees 3 WHERE last_name = 'grant'; no rows selected
Using the LIKE Condition
- Use the LIKE condition to perform wildcard searches of valid search string values.
- Search conditions can contain either literal character or numbers:
- % denotes zero or many characrters.
- _ denotes one character
- You can combine pattern-matching characters
SELECT last_name FROM employees WHERE last_name LIKE '_o%';
- You can use the ESCAPE identifier to search for the actual % and _ symbols.
SELECT employee_id,last_name,job_id FROM employees WHERE job_id LIKE '%SA\_%' ESCAPE '';
Using the NULL Conditions
null value的比较只能使用IS NULL或者IS NOT NULL而不能使用=,<>符号进行比较.