MySQL Interviewfragen und Antworten
Frage 86. How To Add a New Column to an Existing Table in MySQL?
If you have an existing table with existing data rows, and want to add a new column to that table, you can use the "ALTER TABLE ... ADD COLUMN" statement. The tutorial script below shows you a good example:
mysql> ALTER TABLE test ADD COLUMN author VARCHAR(40);
Query OK, 1 row affected (0.18 sec)
Records: 1 Duplicates: 0 Warnings: 0
Frage 87. How To Enter Characters as HEX Numbers?
If you want to enter characters as HEX numbers, you can quote HEX numbers with single quotes and a prefix of (X), or just prefix HEX numbers with (0x). A HEX number string will be automatically converted into a character string, if the expression context is a string. Here are some good examples:
SELECT X'313233' FROM DUAL;
123
SELECT 0x414243 FROM DUAL;
ABC
SELECT 0x46594963656E7465722E636F6D FROM DUAL;
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Frage 88. How To Delete an Existing Column in a Table?
If you have an existing column in a table and you do not need that column any more, you can delete it with "ALTER TABLE ... DROP COLUMN" statement. Here is a tutorial script to delete an existing column:
mysql> ALTER TABLE test DROP COLUMN create_date;
Query OK, 1 row affected (0.48 sec)
Records: 1 Duplicates: 0 Warnings: 0
Frage 89. How To Enter Numeric Values as HEX Numbers?
If you want to enter numeric values as HEX numbers, you can quote HEX numbers with single quotes and a prefix of (X), or just prefix HEX numbers with (0x). A HEX number string will be automatically converted into a numeric value, if the expression context is a numeric value. Here are some good examples:
SELECT X'10' + 16 FROM DUAL;
32
SELECT 0x1000 + 0 FROM DUAL;
4096
Frage 90. How To Rename an Existing Column in a Table?
If you have an existing column in a table and you want to change the column name, you can use the "ALTER TABLE ... CHANGE" statement. This statement allows you to change the name of a column, and its definition. The tutorial script below gives you a good example:
mysql> ALTER TABLE test CHANGE COLUMN subject
title VARCHAR(60);
Query OK, 1 row affected (0.51 sec)
Records: 1 Duplicates: 0 Warnings: 0
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