MySQL 面试题与答案
问题 71. What Are Numeric Data Types?
MySQL supports the following numeric data types:
► BIT(n) - An integer with n bits.
► BOOL same as BOOLEAN - Boolean values stored in 1 bit.
► TINYINT - A small integer stored in 1 byte.
► SMALLINT - A small integer stored in 2 bytes.
► MEDIUMINT - A medium integer stored in 3 bytes.
► INT same as INTEGER - An integer stored in 4 bytes.
► BIGINT - An integer stored in 8 bytes.
► FLOAT - A single-precision floating-point number stored in 4 bytes.
► DOUBLE same as DOUBLE PRECISION - A double-precision floating-point number stored in 8 bytes.
► REAL - Same DOUBLE by default.
► DECIMAL(m,d) - A fixed-point number with m as the precision (total number of digits) and d as the scale (number of digits after the decimal point).
► Date and Time Data Types - DATE, DATETIME, TIMESTAMP, TIME, YEAR.
问题 72. What Are DDL Statements in MySQL?
DDL (Data Definition Language) statements are statements to create and manage data objects in the database. The are 3 primary DDL statements:
► CREATE - Creating a new database object.
► ALTER - Altering the definition of an existing data object.
► DROP - Dropping an existing data object.
问题 73. What Are Date and Time Data Types?
MySQL supports the following date and time data types:
► DATE - A date in the range of '1000-01-01' and '9999-12-31'. Default DATE format is "YYYY-MM-DD".
► DATETIME - A date with the time of day in the range of '1000-01-01 00:00:00' and '9999-12-31 23:59:59'. Default DATETIME format is "YYYY-MM-DD HH:MM:SS".
► TIMESTAMP - A timestamp. The range is '1970-01-01 00:00:00' to partway through the year 2037. Default DATETIME format is "YYYY-MM-DD HH:MM:SS".
► TIME - A time. The range is '-838:59:59' to '838:59:59'. Default TIME format is "HH:MM:SS".
► TIME - A time. The range is '-838:59:59' to '838:59:59'. Default TIME format is "HH:MM:SS".
► YEAR - A year in 4 digits in the range of 1901 and 2155. Default YEAR format is "YYYY".
问题 74. How To Create a New Table in MySQL?
If you want to create a new table, you can use the "CREATE TABLE" statement. The following tutorial script shows you how to create a table called "tip":
mysql> CREATE TABLE tip (id INTEGER PRIMARY KEY,
subject VARCHAR(80) NOT NULL,
description VARCHAR(256) NOT NULL,
create_date DATE NULL);
Query OK, 0 rows affected (0.08 sec)
This scripts creates a testing table called "tip" with 4 columns in the current database.
问题 75. How To Calculate Expressions with SQL Statements?
There is no special SQL statements to calculate expressions. But you can use the "SELECT expression FROM DUAL" statement return the calculated value of an expression. "DUAL" is a dummy table in the server. The tutorial exercise below shows you some good examples:
SELECT 'Hello world!' FROM DUAL;
Hello world!
SELECT (1+2)*3/4 FROM DUAL;
2.2500
SELECT TRUE FROM DUAL;
1
SELECT TRUE AND FALSE FROM DUAL;
0
SELECT TIME(SYSDATE()) FROM DUAL;
21:30:26
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