OCP之黄伟 2
Restricting and Sorting DataObjectives
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
2FROM employees
3WHERE last_name = 'Grant';
LAST_NAME JOB_ID DEPARTMENT_ID
------------------------- ---------- -------------
Grant SH_CLERK 50
Grant SA_REP
SQL> SELECT last_name,job_id,department_id
2FROM employees
3WHERE last_name = 'grant';
no rows selected
View Code
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.
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