SQL 面试题与答案
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问题 116. What is ON DELETE CASCADE?
When ON DELETE CASCADE is specified Oracle maintains referential integrity by automatically removing dependent foreign key values if a referenced primary or unique key value is removed.
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问题 117. What are the data types allowed in a table?
CHAR, VARCHAR2, NUMBER, DATE, RAW, LONG and LONG RAW.
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问题 118. What is difference between CHAR and VARCHAR2? What is the maximum SIZE allowed for each type?
CHAR pads blank spaces to the maximum length.
VARCHAR2 does not pad blank spaces.
For CHAR the maximum length is 255 and 2000 for VARCHAR2.
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问题 119. How many LONG columns are allowed in a table? Is it possible to use LONG columns in WHERE clause or ORDER BY?
Only one LONG column is allowed. It is not possible to use LONG column in WHERE or ORDER BY clause.
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问题 120. What are the pre-requisites to modify datatype of a column and to add a column with NOT NULL constraint?
To modify the datatype of a column the column must be empty.
- To add a column with NOT NULL constrain, the table must be empty.
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