问题
填空题
SQL中修改表结构应该使用的语句是（）。
答案
参考答案：ALTER TABLE
解析：
SQL语言用ALTER TABLE语句修改基本表，其一般格式为
ALTER TABLE＜表名＞[ADD＜新列名＞＜数据类型＞[完整性约束]][DROP＜完整性约束名＞][MODIFY＜列名＞＜数据类型＞]；
其中＜表名＞为指定需要修改的基本表名，ADD子句用于增加新列和新的完整性约束，DROP子句用于删除指定的完整性约束，MODIFY子句用于修改原有的列定义。
import java.applet.*; import java.awt.*; import java.awt.*; /* ＜applet code="ex22_3.class" width=800 height=400 ＞ ＜/applet＞ */ class CanvasCircle extends Canvas{ private Color cCircle; public CanvasCircle(Color c){ cCircle = c; } public void paint(Graphics g){ int dimCricle, xCircle, yCircle; Dimension sizeCurrent = getSize(); if (sizeCurrent.width ＞ sizeCurrent.height){dimCricle = sizeCurrent.width; } else{dimCricle = sizeCurrent.height; } g.setColor(cCircle); xCircle = (sizeCurrent.height - dimCricle) / 2; yCircle = (sizeCurrent.width - dimCricle) / 2; g.fillOval(xCircle, yCircle, dimCricle, dimCricle); } } public class ex22_3 extends Applet{ public void init(){GridBagLayout gbl = new GridBagLayout();GridBagConstraints gbc = new GridBagConstraints();CanvasCircle circleNew;setLayout(gbl);gbc.weightx = 1.0;gbc.weighty = 1.0;gbc.fill = GridBagConstraints.BOTH;circleNew = new CanvasCircle(Color.red);gbl.setConstraints(circleNew, gbc);add(circleNew);gbc.weightx = 1.5;gbc.weighty = 1.5;circleNew = new CanvasCircle(Color.green);gbl.setConstraints(circleNew, gbc);add(circleNew);gbc.weightx = 2.0;gbc.weighty = 2.0;circleNew = new CanvasCircle(Color.blue);gbl.setConstraints(circleNew, gbc);add(circleNew); } } ex22_3.h tml ＜HTML＞ ＜HEAD＞ ＜TITLE＞ex22_3＜/TITLE＞ ＜/HEAD＞ ＜BODY＞ ＜applet code="ex22_3.class" width=800 height=400 ＞ ＜/applet＞ ＜/BODY＞ ＜/HTML＞