Advanced SQL#

Accessing SQL from a Programming Language#

Not all queries can be expressed in SQL, since SQL does not provide the full expressive power of a general-purpose language.
不是所有的查询都能用 SQL 语言表达。
Non-declarative actions -- such as printing a report, interacting with a user, or sending the results of a query to a graphical user interface -- cannot be done from within SQL.
用户交互是图形界面，语音、图像，数据库不具备这方面的功能。

从高级语言（如 C）访问数据库，主要是下面两种方式：

API(Application Program Interface) -- A general-purpose program can connect to and communicate with a database server using a collection of functions.
函数库
Embedded SQL -- provides a means by which a program can interact with a database server.
把 SQL 语句嵌入到语言内
- The SQL statements are translated at compile time into function calls.
- At runtime, these function calls connect to the database using an API that provides dynamic SQL facilities.

JDBC and ODBC#

API (application-program interface) for a program to interact with a database server
Application makes calls to

Connect with the database server
建立连接
Send SQL commands to the database server
指令发送到服务器
Fetch tuples of result one-by-one into program variables
拿回结果

SQL 与 C 语言存在鸿沟（如 select 得到的是集合，但是 C 语言没有这种类型）会返回指针/迭代器

ODBC (Open Database Connectivity) works with C, C++, C#
JDBC (Java Database Connectivity) works with Java
通过类定义，将数据库操作封装到 Java 内
Embedded SQL in C
SQLJ - embedded SQL in Java
JPA(Java Persistence API) - OR mapping of Java

JDBC#

JDBC is a Java API for communicating with database systems supporting SQL.

public static void JDBCexample(String dbid, String userid, String passwd) 
        { 
     try { 
   Connection conn = DriverManager.getConnection("jdbc:oracle:thin:@db.yale.edu:2000:univdb", userid, passwd); 
          Statement stmt = conn.createStatement(); 
              ... Do Actual Work ...
          stmt.close(); 
          conn.close(); 
     }      
     catch (SQLException sqle) {        
          System.out.println("SQLException : " + sqle);     
     }      
        }

Open a connection
Create a “statement” object
Execute queries using the Statement object to send queries and fetch results
Exception mechanism to handle errors

Example

Update to database

Update to database 
try {
    stmt.executeUpdate(
        "insert into instructor values(’77987’, ’Kim’, ’Physics’, 98000)");
        } catch (SQLException sqle)
        {
            System.out.println("Could not insert tuple. " + sqle);
        }

Execute query and fetch and print results

ResultSet rset = stmt.executeQuery(
        "select dept_name, avg (salary)
        from instructor
        group by dept_name");
        while (rset.next()) {
            System.out.println(rset.getString("dept_name") + " " + rset.getFloat(2));
        }

Getting result fields:

rset.getString(“dept_name”) and rset.getString(1) equivalent if dept_name is the first argument of select result.

Dealing with Null values

int a = rset.getInt(“a”);
    if (rset.wasNull()) Systems.out.println(“Got null value”);

Prepared Statement

Prepared Statement

PreparedStatement pStmt = conn.prepareStatement(
        "insert into instructor values(?,?,?,?)");
pStmt.setString(1, "88877");      
pStmt.setString(2, "Perry");
pStmt.setString(3, "Finance");   
pStmt.setInt(4, 125000);
pStmt.executeUpdate();    
pStmt.setString(1, "88878");
pStmt.executeUpdate();

这里空格是占位符，表示执行时需要提供四个参数。
setString, setInt 就是把第几个占位符设置为参数，并 executeUpdate 进行插入。

SQL Injection

Always use prepared statements when taking an input from the user and adding it to a query. NEVER create a query by concatenating strings which you get as inputs.
SQL 注入攻击。

Suppose query is constructed using select * from instructor where name = ’" + name + “ ’
Suppose the user, instead of entering a name, enters: X’ or ’Y’ = ’Y
then the resulting string of the statement becomes: select * from instructor where name = ’" + "X’ or ’Y’ = ’Y" + “’
which is: select * from instructor where name = ’X’ or ’Y’ = ’Y’
User could have even used
X’; update instructor set salary = salary + 10000;
then select * from instructor where name = ’X’; update instructor set salary = salary + 10000;
Always use prepared statements, with user inputs as parameters

Metadata Features

ResultSet metadata

Example

after executing query to get a ResultSet rs:

ResultSetMetaData rsmd = rs.getMetaData();
for(int i = 1; i <= rsmd.getColumnCount(); i++) {
    System.out.println(rsmd.getColumnName(i));
            System.out.println(rsmd.getColumnTypeName(i));
    }

Database metadata

Example

DatabaseMetaData dbmd = conn.getMetaData();
ResultSet rs = dbmd.getColumns(null, "univdb", "department", "%");
// Arguments to getColumns: Catalog, Schema-pattern, Table-pattern,
// and Column-Pattern
// Returns: One row for each column; row has a number of attributes
// such as COLUMN_NAME, TYPE_NAME
while( rs.next()) {
    System.out.println(rs.getString("COLUMN_NAME"),
                                                    rs.getString("TYPE_NAME");

Transaction Control in JDBC

Can turn off automatic commit on a connection conn.setAutoCommit(false);
Transactions must then be committed or rolled back explicitly conn.commit(); or conn.rollback();
conn.setAutoCommit(true) turns on automatic commit.

所有的数据库功能都是通过 Java 封装好的类来实现的。

SQLJ#

SQLJ: embedded SQL in Java

#sql iterator deptInfoIter ( String dept name, int avgSal);
    deptInfoIter iter = null;
    #sql iter = { select dept_name, avg(salary) as avgSal from instructor
             group by dept name };
    while (iter.next()) {
           String deptName = iter.dept_name();
          int avgSal = iter.avgSal();
          System.out.println(deptName + " " + avgSal);
    }
    iter.close();

嵌入都要 #sql 标识，最后会被编译器转化为 Java 的类。

ODBC#

Each database system supporting ODBC provides a "driver" library that must be linked with the client program.

Example

int ODBCexample()
{
    RETCODE error;
    HENV    env;     /* environment */ 
    HDBC    conn;  /* database connection */ 
    SQLAllocEnv(&env);
    SQLAllocConnect(env, &conn);
    SQLConnect(conn, “db.yale.edu", SQL_NTS, "avi", SQL_NTS, "avipasswd", SQL_NTS); 
    { ... Do actual work ... }

    SQLDisconnect(conn); 
    SQLFreeConnect(conn); 
    SQLFreeEnv(env); 
}

同一个数据库可能服务于多个用户，而且使用的编程语言可能不同，如字符串的结束标志可能也不同，因此需要用 SQL_NTS 标识。

Program sends SQL commands to database by using SQLExecDirect
Result tuples are fetched using SQLFetch()
SQLBindCol() binds C language variables to attributes of the query result
- When a tuple is fetched, its attribute values are automatically stored in corresponding C variables.
- Arguments to SQLBindCol()
  - ODBC stmt variable, attribute position in query result
  - The type conversion from SQL to C.
  - The address of the variable.
  - For variable-length types like character arrays,
    - The maximum length of the variable
    - Location to store actual length when a tuple is fetched.
    - Note: A negative value returned for the length field indicates null value

Example

Main body of program
char deptname[80];
float salary;
int lenOut1, lenOut2;
HSTMT stmt;
char * sqlquery = "select dept_name, sum (salary) from instructor group by dept_name";
SQLAllocStmt(conn, &stmt);
error = SQLExecDirect(stmt, sqlquery, SQL_NTS);
if (error == SQL SUCCESS) {
    SQLBindCol(stmt, 1, SQL_C_CHAR, deptname , 80, &lenOut1);
    SQLBindCol(stmt, 2, SQL_C_FLOAT, &salary, 0 , &lenOut2);
    while (SQLFetch(stmt) == SQL_SUCCESS) {
        printf (" %s %g\n", deptname, salary);
    }
}
SQLFreeStmt(stmt, SQL_DROP);

定义数组需要多一个，否则会有截断。如 char deptname[11]; 才能定义十个元组。
如果结果为空，则 lenOut 为 -1.

ODBC Prepared Statements

SQL statement prepared: compiled at the database
- To prepare a statement SQLPrepare(stmt, <SQL String>);
- To bind parameters SQLBindParameter(stmt, <parameter#>, ... type information and value omitted for simplicity..)
- To execute the statement retcode = SQLExecute(stmt);
Can have placeholders: e.g. insert into account values(?,?,?)
Repeatedly executed with actual values for the placeholders

More ODBC Features

Metadata features
- finding all the relations in the database and
- finding the names and types of columns of a query result or a relation in the database.
By default, each SQL statement is treated as a separate transaction that is committed automatically.
- Can turn off automatic commit on a connection SQLSetConnectOption(conn, SQL_AUTOCOMMIT, 0)}
- Transactions must then be committed or rolled back explicitly by SQLTransact(conn, SQL_COMMIT) or SQLTransact(conn, SQL_ROLLBACK)

Embedded SQL#

A language to which SQL queries are embedded is referred to as a host language, and the SQL structures permitted in the host language comprise embedded SQL.
如把 SQL 嵌入到 C 语言，那么 C 语言是 host.

在编译前，有一个预编译器，将 SQL 语句翻译。

EXEC SQL statement is used in the host language to identify embedded SQL request to the preprocessor (in Java, # SQL { ... };)

Issues with Embedded SQL

Mark the start point and end point of Embedded SQL EXEC SQL <statement>； //C
Communication between database and programming language e.g. SQLCA、SQLDA
Address the mismatching issue between SQL and host lanugage.
Handle result (set) with cursor Mapping of basic data types e.g. SQL: Date \(\rightarrow\) C: char(12)

Example

insert、delete、update、select(single record)
main( )
{  EXEC SQL INCLUDE SQLCA; //声明段开始
    EXEC SQL BEGIN DECLARE SECTION;
    char account_no [11];    //host variables(宿主变量)声明
    char branch_name [16];
    int  balance;  
EXEC SQL END DECLARE SECTION;//声明段结束
EXEC SQL CONNECT  TO  bank_db  USER Adam Using Eve; 
scanf (“%s  %s  %d”, account_no, branch_name, balance);
EXEC SQL insert into account 
                values (:account_no, :branch_name, :balance);
If (SQLCA.sqlcode ! = 0)    printf ( “Error!\n”);
else       printf (“Success!\n”);
}

最开始声明段中的 host 语句，可以用在 SQL 语句里。

两点不平衡：没有集合；没有 NULL；没有日期类型

可以在编译时进行类型检查，但 ODBC 只有在运行时才有。

Static： Embedded SQL statements( include relation names and attribute names) are hard coded in program.
Dynamic：Embedded SQL statements are built at run time

Procedural Constructs in SQL#

SQL provides a module language
Permits definition of procedures in SQL, with if-then-else statements, for and while loops, etc.

Stored Procedures

Can store procedures in the database
then execute them using the call statement
permit external applications to operate on the database without knowing about internal details

SQL Functions#

Example

SQL 函数的返回值可以是一个 table.

Example

SQL Procedures#

有输入参数(in)和输出参数(out)

Example

Procedural Constructs#

Compound statement: begin ... end,

May contain multiple SQL statements between begin and end.
Local variables can be declared within a compound statements

While and repeat statements
e.g.

declare n integer default 0;
while n < 10 do
    set n = n + 1
end while                           
        repeat
    set n = n  – 1
        until n = 0
end repeat

For loop Permits iteration over all results of a query e.g.

declare n  integer default 0;
for r as
    select budget from department 
    where dept_name = ‘Music’ 
    do 
        set n = n - r.budget 
end for

r 表示返回的每一行

Example procedure

External Language Functions/Procedures#

SQL 可以访问由 C 语言定义的函数（过程）

Example

create procedure dept_count_proc(in dept_name varchar(20),  count integer)
language C 
external name ’ /usr/avi/bin/dept_count_proc’ 
create function dept_count(dept_name varchar(20))
returns integer
language C
external name ‘/usr/avi/bin/dept_count’

可能比较危险，放在虚拟机（Java）或者独立的线程

Triggers#

A trigger is a statement that is executed automatically by the system as a side effect of a modification to the database.

Trigger - ECA rule

E: Event （ insert, delete ，update）
C: Condition
A: Action

To design a trigger mechanism, we must:

Specify the conditions under which the trigger is to be executed.
Specify the actions to be taken when the trigger executes.

Example

time_slot_id Example

这里 time_slot_id 不是主键，因此删除不会引起其他影响。但我们可以设计一个触发器，用来检查当前课程的 time_slot_id 是否在表内。
第二个触发器表示，time_slot_id 已经被删完了，但依然有课程在引用，就要 rollback.

Triggering event can be insert, delete or update
Triggers on update can be restricted to specific attributes
e.g. after(before) update of takes on grade
Values of attributes before and after an update can be referenced
- referencing old row as: for deletes and updates
- referencing new row as: for inserts and updates

Trigger to Maintain credits_earned value

如果本来挂科，或者没有成绩，更新后不再挂科而且有成绩，就把学分加上去。

要慎用触发器，用在刀刃上，可能会引发连锁反应。

Instead of executing a separate action for each affected row, a single action can be executed for all rows affected by a transaction

Use for each statement instead of for each row
Use referencing old table or referencing new table to refer to temporary tables (called transition tables) containing the affected rows
Can be more efficient when dealing with SQL statements that update a large number of rows

Example

Last update: 2024年3月19日 11:22:29
Created: 2024年3月19日 11:22:29