MySQL preguntas y respuestas de entrevista
Pregunta 81. How To Escape Special Characters in SQL statements?
There are a number of special characters that needs to be escaped (protected), if you want to include them in a character string. Here are some basic character escaping rules:
► The escape character () needs to be escaped as ().
► The single quote (') needs to be escaped as (') or ('') in single-quote quoted strings.
► The double quote (") needs to be escaped as (") or ("") in double-quote quoted strings.
► The wild card character for a single character (_) needs to be escaped as (_).
► The wild card character for multiple characters (%) needs to be escaped as (%).
► The tab character needs to be escaped as (t).
► The new line character needs to be escaped as (n).
► The carriage return character needs to be escaped as (r).
Here are some examples of how to include special characters:
SELECT 'It''s Sunday!' FROM DUAL;
It's Sunday!
SELECT 'Allo, C'est moi.' FROM DUAL;
Allo, C'est moi.
SELECT 'MontTuetWedtThutFri' FROM DUAL;
Mon Tue Wed Thu Fri
Pregunta 82. How To See the CREATE TABLE Statement of an Existing Table?
SHOW CREATE TABLE test;
Pregunta 83. How To Concatenate Two Character Strings?
If you want concatenate multiple character strings into one, you need to use the CONCAT() function. Here are some good examples:
SELECT CONCAT('Welcome',' to') FROM DUAL;
Welcome to
SELECT CONCAT('wb','center','.com') FROM DUAL;
WithoutBook.com
Pregunta 84. How To Create a New Table by Selecting Rows from Another Table in MySQL?
Let's say you have a table with many data rows, now you want to create a backup copy of this table of all rows or a subset of them, you can use the "CREATE TABLE ... SELECT" statement. The tutorial script below gives you a good example:
mysql> INSERT INTO tip VALUES (1, 'Learn MySQL',
'Visit www.GlobalGuideLine.com','2006-07-01');
Query OK, 1 row affected (0.62 sec)
mysql> CREATE TABLE tipBackup SELECT * FROM tip;
Query OK, 1 row affected (0.49 sec)
Records: 1 Duplicates: 0 Warnings: 0
Pregunta 85. How To Include Numeric Values in SQL statements?
If you want to include a numeric value in your SQL statement, you can enter it directly as shown in the following examples:
SELECT 255 FROM DUAL; -- An integer
255
SELECT -6.34 FROM DUAL; -- A regular number
-6.34
SELECT -32032.6809e+10 FROM DUAL; -- A floating-point value
-3.20326809e+014
Lo mas util segun los usuarios: