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Module 6: Applications Programming with Dynamic SQL

Dynamic Embedded SQL (in C)

Dynamic SQL

A protocol for using SQL within another language is dynamic if it enables arbitrary SQL source code to be constructed and executed at run-time.

Dynamic Embedded SQL: a Roadmap

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EXECUTE IMMEDIATE

To execute a non-parametric statement, use the command

\[ \texttt{ EXEC SQL EXECUTE IMMEDIATE :string; } \]

where :string is a host variable containing the ASCII representation of the command.

  • :string may not be a command that returns answers, such as a query.
  • EXECUTE IMMEDIATE should only be used for commands that will be executed only once by an app.
    • :string is compiled each time control flow reaches the command

PREPARE

Run-Time Compilation

The SQL PREPARE command is used at run-time to compile SQL commands and to provide a handle to the generated code.

To compile, use the command

\[ \texttt{EXEC SQL PREPARE stmt FROM :string;} \]

Code generated for :string can now be invoked via the stmt handle in other SQL commands.

  • enables recompilation of :string to be avoided
  • :string may now contain parameters and be a query that returns answers.

  • The stmt handle is not a host variable, and refers to generated code for :string that will not be reentrent.

    Note

    ❗

    Concept Explanation from ChatGPT:

    The phrase "generated code for :string that will not be reentrant" means that the compiled or prepared SQL statement (:string) stored in the stmt handle cannot be safely reused in multiple, overlapping contexts (such as recursion or concurrent executions).

    • statement handles should not be used in recursion

Parametric Statements

Parameter Markers

ASCII representations of SQL commands can introduce "?" characters as parameter markers for locations of parameter values that will be substituted when the compiled commands are executed.

EXECUTE

To execute a prepared SQL command with parameter markers via a statement handle, use the EXECUTE command with the USING clause.

\[ \texttt{EXEC SQL EXECUTE stmt USING :var1 [,...,:vark];} \]

Used for commands that do not return tuples.

  • database modification (INSERT, . . . )
  • transaction management (COMMIT)
  • data definition(CREATE TABLE,...)

Values of host variables :var1 through :vark are substituted for the parameter markers in order of appearance.

  • a mismatch will cause an SQL runtime error

Using SQL Cursors

To execute a prepared SQL query with parameter markers and a statement handle, use the USING clause with the OPEN command and follow the cursor iterator protocol.

EXEC SQL DECLARE cname CURSOR FOR stmt;
EXEC SQL OPEN  cname
         USING :var1 [,...,:vark];
EXEC SQL FETCH cname
         INTO  :out1 [,...,:outn];
EXEC SQL CLOSE cname;

Host variables :var1 through :vark supply values for parameter markers.

Host variables :out1 through :outn store values for a retrieved tuple.

The number of retrieved tuples is defined in sqlca.sqlerrd[2].

When Parameters and Results are not Known

DESCRIPTOR

An SQL descriptor is used to communicate information about parameters and results of a prepared SQL command.

SQL commands for descriptor use:

ALLOCATE DESCRIPTOR descr
GET DESCRIPTOR descr <what>
SET DESCRIPTOR descr <what>

where <what> indicates

  1. GET/SET a value for COUNT, or
  2. GET/SET properties for the \(i^{th}\) attribute: VALUE :i <prop> where <prop> can be DATA, TYPE, INDICATOR, . . .

DESCRIBE [INPUT | OUTPUT] stmt INTO descr

The SQLDA Data Structure

The \(\texttt{sqlda}\) data structure is an SQL description area that defines what attributes that are parameters and query answers look like, e.g., where the data is located.

This is how the RDBMS communicates with an application.

The data structure contains (among other things):

  1. the string ’SQLDA ’ (for identification);
  2. the number of allocated entries for attributes;
  3. the number of actual attributes, 0 if none; and
  4. for each attribute:
    1. a numeric code of its type,
    2. the length of storage for its value,
    3. a pointer to a data variable,
    4. a pointer to a indicator variable, and
    5. its name (a string and a length).

SQLDA ala DB2

struct sqlname /* AttributeName */
{
    short length; /* Name length [1..30] */
    char data[30]; /* Variable or Column name */
};
struct sqlvar /* Attribute Descriptor */
{
    short sqltype; /* Variable data type */
    short sqllen; /* Variable data length */
    char *SQL_POINTER sqldata; /* data buffer */
    short *SQL_POINTER sqlind; /* null indiciator */
    struct sqlname sqlname; /* Variable name */
};
struct sqlda /* Main SQLDA */
{
    char sqldaid[8]; /* Eye catcher = ’SQLDA ’ */
    long sqldabc; /* SQLDA size in bytes=16+44*SQLN */
    short sqln; /* Number of SQLVAR elements */
    short sqld; /* Number of used SQLVAR elements */
    struct sqlvar sqlvar[1]; /* first SQLVAR element */
};

SQLDA ala ORACLE6

struct SQLDA {
    long N; /* Descriptor size in number of entries */
    char *V[]; /* Arr of addresses of main variables (data) */
    long L[]; /* Arr of lengths of data buffers */
    short T[]; /* Arr of types of buffers */
    short *I[]; /* Arr of addresses of indicator vars */
    long F; /* Number of variables found by DESCRIBE */
    char *S[]; /* Arr of variable name pointers */
    short M[]; /* Arr of max lengths of attribute names */
    short C[]; /* Arr of current lengths of attribute names */
    char *X[]; /* Arr of indicator name pointers */
    short Y[]; /* Arr of max lengths of ind. names */
    short Z[]; /* Arr of cur lengths of ind. names */
};

DESCRIBE via SQLDA

A prepared statement can be described via an \(\texttt{sqlda}\) data structure with the SQL DESCRIBE command.

\[ \texttt{ EXEC SQL DESCRIBE stmt INTO :sqlda } \]

The result is:

  • the number of result attributes
    • 0 when not a query
  • for every attribute in an answer tuple when stmt is a query:
    1. its name and length, and
    2. its type.

SQLDA and Parameter Passing

One uses an SQLDA descriptor to also supply parameters.

Descriptors can therefore substitute host variables as the argument of the SQL USING clause.

EXEC SQL EXECUTE stmt
         USING DESCRIPTOR :sqlda;
EXEC SQL OPEN cname
         USING DESCRIPTOR :sqlda;
EXEC SQL FETCH cname
         USING DESCRIPTOR :sqlda;

An Example Application: \(\texttt{adhoc.sqc}\)

\(\texttt{adhoc}\) is an application that executes an SQL statement provided as its argument on the command line.

// Declarations:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "util.h"

EXEC SQL INCLUDE SQLCA;
EXEC SQL INCLUDE SQLDA;

EXEC SQL BEGIN DECLARE SECTION;
   char  db[6] = "cs348";
   char  sqlstmt[1000];
EXEC SQL END DECLARE SECTION;

struct sqlda *slct;

// Start up and prepare the statement:
int main(int argc, char *argv[]) {
   int i, isnull; short type;

   printf("Sample C program : ADHOC interactive SQL\n");

   /* bail out on error */
   EXEC SQL WHENEVER SQLERROR  GO TO error;

   /* connect to the database */
   EXEC SQL CONNECT TO :db;
   printf("Connected to DB2\n");

   strncpy(sqlstmt,argv[1],1000);
   printf("Processing <%s>\n",sqlstmt);

   /* compile the sql statement */
   EXEC SQL PREPARE stmt FROM :sqlstmt;

   init_da(&slct,1);

   /* now we find out what it is */
   EXEC SQL DESCRIBE stmt INTO :*slct;

   i= slct->sqld;
   // ...its a query:
   if (i>0) {
     printf("      ... looks like a query\n");

     /* new SQLDA to hold enough descriptors for answer */
     init_da(&slct,i);

     /* get the names, types, etc... */
     EXEC SQL DESCRIBE stmt INTO :*slct;

     printf("Number of variables <%d>\n",slct->sqld);
     for (i=0; i<slct->sqld; i++ ) {
       printf("  variable %d <%.*s (%d%s [%d])>\n",
                    i,
                    slct->sqlvar[i].sqlname.length,
                    slct->sqlvar[i].sqlname.data,
                    slct->sqlvar[i].sqltype,
                    ( (slct->sqlvar[i].sqltype&1)==1 ? "": " not null"),
                    slct->sqlvar[i].sqllen);
     }
     printf("\n");

     // ... more processing for queries: prepare buffers and print a header.
     /* allocate buffers for the returned tuples */
     for (i=0; i<slct->sqld; i++ ) {
       slct->sqlvar[i].sqldata = malloc(slct->sqlvar[i].sqllen);
       slct->sqlvar[i].sqlind = malloc(sizeof(short));
       *slct->sqlvar[i].sqlind = 0;
     }

         // ... more processing for queries: fetch and print answers.
     /* and now process the query */
     EXEC SQL DECLARE cstmt CURSOR FOR stmt;
     EXEC SQL OPEN cstmt;
     EXEC SQL WHENEVER NOT FOUND GO TO end;

     /* print the header */
     for (i=0; i<slct->sqld; i++ )
       printf("%-*.*s ",slct->sqlvar[i].sqllen,
                        slct->sqlvar[i].sqlname.length,
                        slct->sqlvar[i].sqlname.data);
     printf("\n");

     for (;;) {
       /* fetch next tuple into the prepared buffers */
       EXEC SQL FETCH cstmt USING DESCRIPTOR :*slct;
       for (i=0; i<slct->sqld; i++ )
         if ( *(slct->sqlvar[i].sqlind) < 0 )
           print_var("NULL",
                     slct->sqlvar[i].sqltype,
                     slct->sqlvar[i].sqlname.length,
                     slct->sqlvar[i].sqllen);
         else
           print_var(slct->sqlvar[i].sqldata,
                     slct->sqlvar[i].sqltype,
                     slct->sqlvar[i].sqlname.length,
                     slct->sqlvar[i].sqllen);
       printf("\n");
       };
   end:
     printf("\n");
     // ... otherwise it is a simple statement: just execute it.
   } else {
     printf("      ... looks like an update\n");

     EXEC SQL EXECUTE stmt;
   };
   //printf("Rows processed: %d\n",sqlca.sqlerrd[2]);

   /* and get out of here */
   EXEC SQL COMMIT;
   EXEC SQL CONNECT reset;
   exit(0);

error:
   check_error("My error",&sqlca);
   EXEC SQL WHENEVER SQLERROR CONTINUE;

   EXEC SQL ROLLBACK;
   EXEC SQL CONNECT reset;
   exit(1);
}

Dynamic Embedded SQL Summary

  • Given a string:
    1. if nothing known, use DESCRIBE;
    2. if a simple statement used once, use EXECUTE IMMEDIATE;
    3. otherwise use PREPARE.
  • Given a statement handle obtained via PREPARE:
    1. if a simple statement used once, use EXECUTE;
    2. for the query otherwise, use DECLARE CURSOR and then process as an ordinary cursor.

ODBC (Open Database Connectivity)

ODBC: An SQL Call Level Interface (CLI)

  • Overview:
    • An interface defined by a C library of function calls developed by Microsoft.
  • Development:
    • Applications are developed entirely in a host language.
    • No pre-compilation so no type checking at compile time.
    • Less transparent compared to static embedded SQL.
  • Standards:
    • The SQL CLI standard incorporates ODBC and X/Open standards.
  • ODBC Program Handles:
    1. Environment: Exactly one per application thread.
    2. Database Engine Connection.
    3. SQL Statement.
  • Execution:
    • Dynamic embedded SQL has statement handles only.
    • An executing thread has at most one ongoing transaction.

Connect and Disconnect in ODBC

int main()
{
    SQLHENV henv;
    SQLHDBC hdbc;
    SQLRETURN rc;
    SQLCHAR server[SQL_MAX_DSN_LENGTH + 1] = "DBCLASS";
    SQLCHAR uid[19] = "<your uid>";
    SQLCHAR pwd[31] = "<your password>";

    SQLAllocEnv(&henv);
    SQLAllocConnect(henv, &hdbc);

    rc = SQLConnect(hdbc, server, SQL_NTS, uid, SQL_NTS, pwd, SQL_NTS);
    if (rc != SQL_SUCCESS) {
        printf("Error connecting to %s\n", server); exit(1);
    } else printf("Connected to %s\n", server);

    /* DO SOMETHING HERE */

    SQLDisconnect(hdbc);
    SQLFreeConnect(hdbc);
    SQLFreeEnv(henv);
}

Exception Handling in ODBC (errors)

  • Error Handling:
    • Each SQLxxx function returns an error code, similar to libc functions.
    • Error codes should be checked after each SQLxxx call.
  • Return Codes:
    1. SQL_SUCCESS
    2. SQL_ERROR
  • Details:
    • The SQLError function is used to obtain more details about errors.

SQL Statements in ODBC

  • SQLAllocStmt - Allocates a statement handle.
  • SQLExecDirect - Executes a statement directly.
  • SQLPrepare - Compiles the SQL statement.
  • SQLExecute - Executes a compiled statement.
  • SQLSetParam - Initializes a procedure parameter.
  • SQLNumResultCols - Retrieves the number of columns in the result set.
  • SQLBindCol - Associates a column in a result set with a variable in the application.
  • SQLGetData - Retrieves data for a single column in the result set.
  • SQLFetch - Fetches the next row of data.
  • SQLError - Retrieves diagnostic information.
  • SQLRowCount - Gets the number of rows affected by an update, insert, or delete operation.
  • SQLFreeStmt - Frees a statement handle.

Parameters

  1. Parameter markers:
    • Specified by ? in the text of the query
      • SQLNumParams
      • SQLBindParameter
  2. Results of queries:
    • Specified by the number of resulting columns
      • SQLNumResultsCol
      • SQLDescribeCol
      • SQLBindCol or SQLGetData
  3. Number of affected tuples (updates):
    • SQLRowCount

The parameters are bound to buffers by SQLBindParameter.

SQLBindParameter(stmt-handle, param-nr, inp/out, c-type, db-type, db-prec, db-scale, val-ptr, val-len, val-NULL-ptr)

Substitutes a value pointed to by val-ptr (with length val-len, indicator variable val-NULL-ptr, and C data type c-type) for the param-nr-th parameter of stmt-handle (using database type db-type).

Note

❗

Concept Explanation from ChatGPT:

Here's a detailed explanation of each parameter in the SQLBindParameter function, which is used in ODBC (Open Database Connectivity) to bind application variables to SQL parameters in prepared statements:

  1. stmt-handle: This is the handle to the statement associated with the specific SQL query for which parameters are being bound. It identifies the particular statement to be executed.
  2. param-nr: Specifies the number of the parameter. This is a one-based index indicating the position of the parameter in the SQL statement.
  3. inp/out: Indicates the type of parameter in terms of input/output. It can be:
    • SQL_PARAM_INPUT: The parameter is an input parameter.
    • SQL_PARAM_OUTPUT: The parameter is an output parameter.
    • SQL_PARAM_INPUT_OUTPUT: The parameter can be used for both input and output.
  4. c-type: Defines the C data type of the application parameter. For example, SQL_C_CHAR for character data or SQL_C_INT for integers. This parameter tells the driver what kind of C data type is used in the program to represent the SQL data.
  5. db-type: Specifies the SQL data type as defined in the database. For example, SQL_INTEGER or SQL_VARCHAR. This is the data type of the corresponding parameter in the SQL query.
  6. db-prec (Database Precision): Represents the precision of the column data type if applicable. For numeric data types, this would indicate the number of digits.
  7. db-scale: Refers to the scale of the column data type, applicable primarily to fractional numbers. For example, in a column defined as NUMERIC(10,2), the scale is 2.
  8. val-ptr: Pointer to the buffer where the data for the parameter is stored. This buffer holds the actual data to be sent to or received from the database.
  9. val-len: Length of the data in the buffer pointed to by val-ptr. It may also indicate the maximum length of the buffer for output parameters.
  10. val-NULL-ptr: Pointer to a variable that indicates whether the value of the parameter is null. This can be a pointer to an integer or a small variable where SQL_NULL_DATA can be placed if no actual data is to be sent for a nullable parameter.

Example

SQLCHAR stmt[] = "UPDATE author SET url = ? WHERE aid = ?";

SQLINTEGER aid;
SQLCHAR s[70];
SQLINTEGER ind;

rc = SQLAllocStmt(hdbc, &hstmt);

rc = SQLPrepare(hstmt, stmt, SQL_NTS);

printf"(Enter Author ID: "); scanf("%ld",&aid);
printf"(Enter Author URL: "); scanf("%s", s);

rc = SQLBindParameter(hstmt, 1, SQL_PARAM_INPUT, SQL_C_CHAR, SQL_CHAR, 0, 0, s, 70, &ind);
rc = SQLBindParameter(hstmt, 2, SQL_PARAM_INPUT, SQL_C_SLONG, SQL_INTEGER, 0, 0, &aid, 0, NULL);
rc = SQLExecute(hstmt);

Late Binding of Parameters

  • Executing a statement without binding all parameters can be accomplished by setting the last argument of SQLBindParameter to point to the constant SQL_DATA_AT_EXEC.
    • This results in an SQL_NEED_DATA error code.
  • Parameter values can be supplied when detecting this error code by repeated use of two functions:
    • SQLParamData (to determine which argument)
    • SQLPutData (to supply data)
  • Allows updating several rows using one update statement, but supplying different values for each individual row to be updated.

Answers in OBDC

A number of functions exists to obtain output values computed by an SQL statement.

  • Number of affected rows:
    • SQLRowCount
  • Answers to queries:
    • SQLBindCol (to bind variables before execution)
    • SQLGetData (to get values after execution)
  • Obtaining the next tuple of a query evaluation:
    • SQLFetch
      • the result of SQLFetch is just a result code

A Query with SQLBindCol

SQLCHAR sqlstmt[] = "SELECT pubid, title FROM publication";
SQLINTEGER rows;
struct {  SQLINTEGER ind;
                    SQLCHAR s[70];
                    } pubid, title;
rc = SQLAllocStmt(hdbc, &hstmt);
rc = SQLExecDirect(hstmt, sqlstmt, SQL_NTS);
rc = SQLBindCol(hstmt, 1, SQL_C_CHAR, (SQLPOINTER)pubid.s, 8, &pubid.ind);
rc = SQLBindCol(hstmt, 2, SQL_C_CHAR, (SQLPOINTER)title.s, 70, &title.ind);
while ((rc = SQLFetch(hstmt)) == SQL_SUCCESS)
    printf("%-8.8s %-70.70s\n", pubid.s, title.s);
rc = SQLRowCount(hstmt, &rows);
printf(" %d rows selected\n", rows);
rc = SQLFreeStmt(hstmt, SQL_DROP);

A Query with SQLGetData

SQLCHAR sqlstmt[] = "SELECT pubid, title FROM publication";
SQLINTEGER rows;
struct {  SQLINTEGER ind;
                    SQLCHAR s[70];
                    } pubid, title;
rc = SQLAllocStmt(hdbc, &hstmt);
rc = SQLExecDirect(hstmt, sqlstmt, SQL_NTS);
while ((rc = SQLFetch(hstmt)) == SQL_SUCCESS) {
    rc = SQLGetData(hstmt, 1, SQL_C_CHAR, (SQLPOINTER) pubid.s, 8, &(pubid.ind));
    rc = SQLGetData(hstmt, 2, SQL_C_CHAR, (SQLPOINTER) title.s, 70, &(title.ind));
    printf("%-8.8s %-70.70s \n", pubid.s, title.s);
}
rc = SQLRowCount(hstmt, &rows);
printf(" %d rows selected\n", rows);
rc = SQLFreeStmt(hstmt, SQL_DROP);

Column Descriptions in ODBC

There are functions to determine the number of results columns for queries, and column name and type information for each.

SQLNumResultCols(hstmt, &num)
SQLDescribeCol(hstmt, ColNo, ColNamebuf, sizeof(ColNamebuf), NULL, &sqltype, &sqlprec, &sqlscale, &ifNullable);

Example

SQLINTEGER sqlprec;
SQLSMALLINT i, num, sqltype, sqlscale, nullable;
SQLCHAR name[32];

rc = SQLNumResultCols(hstmt, &num);

for (i=0; i<num; i++) {
    rc = SQLDescribeCol(hstmt, i+1, name, 32, NULL, &sqltype, &sqlprec, &sqlscale, &nullable);
    printf("attribute %d is %s (%d,%ld,%d,%d)\n", i, name, sqltype, sqlprec, sqlscale, nullable);
}

Transaction Management in ODBC

  • The start of a new transaction happens implicitly with executable SQL commands:
    • SQLPrepare
    • SQLExecute
    • SQLExecDirect, etc.
  • A function is called to signal the end of a transaction:
    • SQLTransact(henv, hdbc, <what>) where
      • <what> = SQL_COMMIT or
      • <what> = SQL_ROLLBACK

ODBC Summary

  • Expressiveness: ODBC is at least as expressive as dynamic embedded SQL.
    • Not bound to a particular RDBMS engine.
    • Single threads can access multiple RDBMS engines.
  • Statement Dynamics: All statements are dynamic.
    • No pre-compilation is required.
    • Explicit binding of parameters is necessary (user is responsible for ensuring types match).
  • Standardization: ODBC is an almost standard, incorporating ODBC and X/Open standards.
    • Supported by almost all RDBMS engines.
    • Offers hundreds of functions.

Note

❗

Concept Explanation from ChatGPT:

Embedded SQL and Dynamic SQL both allow SQL statements to be integrated into application programs, but they are used in different scenarios and have distinct advantages. Here’s a breakdown of each, including when to use them.

1. Embedded SQL

Embedded SQL refers to SQL code written directly within the host programming language (e.g., C, Java, COBOL) and preprocessed by the compiler before execution. The SQL statements are typically static, meaning they are fixed at compile time.

Advantages of Embedded SQL:

  • Performance: Embedded SQL is often more performant because SQL statements are compiled along with the program code, allowing the database to optimize these queries in advance.
  • Type Safety: SQL statements are checked at compile time, catching syntax errors early, which can reduce runtime errors.
  • Simplified Development: The database structure and operations are predefined, making it easier for developers to understand and manage the code.
  • Security: Static embedded SQL reduces SQL injection risks because queries are predefined and not influenced by user input.

When to Use Embedded SQL:

  • Fixed or Known Queries: When SQL queries are known ahead of time and do not need to change based on user input or other variables, embedded SQL is a good choice.
  • High Performance is Essential: When performance is critical, and static optimization can significantly benefit, embedded SQL provides an advantage.
  • Applications with a Stable Database Schema: In applications where the database schema does not change frequently, embedded SQL works well as there is no need to adjust queries at runtime.

2. Dynamic SQL

Dynamic SQL allows SQL statements to be constructed and executed at runtime, enabling more flexibility than embedded SQL. With dynamic SQL, you can build queries based on user input or program logic, making it ideal for situations where the exact SQL statement may not be known until runtime.

Advantages of Dynamic SQL:

  • Flexibility: Dynamic SQL allows queries to be generated dynamically, making it possible to handle a variety of SQL queries or schema changes at runtime.
  • User-Driven Customization: Allows SQL statements to be tailored based on user inputs, making it suitable for applications where users can define custom queries.
  • Adaptability to Schema Changes: Dynamic SQL can adjust to changes in database structure more readily since the SQL statements are created as needed.
  • Advanced Use Cases: Useful in applications requiring complex conditional logic, user-defined queries, or parameterized data sources.

When to Use Dynamic SQL:

  • Variable or Unknown Queries: When the queries are dependent on user input or vary based on runtime conditions, dynamic SQL is a good fit.
  • Dynamic Reporting and Analytics: For applications that allow users to generate ad-hoc reports or analyses, dynamic SQL provides the flexibility needed.
  • Frequent Schema Changes: In environments where the database schema changes frequently, dynamic SQL can be adjusted easily without recompiling the entire program.
  • Complex Queries with Conditional Logic: When the SQL query structure is complex and depends on multiple conditions evaluated at runtime, dynamic SQL is beneficial.

Summary Comparison

Feature Embedded SQL Dynamic SQL
Performance Higher, due to compile-time optimizations Lower, as queries are compiled at runtime
Flexibility Limited to predefined, static queries High, queries can be constructed dynamically
Security Generally safer from SQL injection Requires careful handling to prevent SQL injection
Compile-time Checking Yes, syntax checked at compile time No, errors may only appear at runtime
Use Cases Fixed or known queries, stable schema Ad-hoc queries, reporting, schema changes

In summary:

  • Use Embedded SQL when queries are known in advance and performance or compile-time safety is crucial.
  • Use Dynamic SQL when queries need to be flexible, user-defined, or depend on conditions that are only known at runtime.

Note

❗

Concept Explanation from ChatGPT:

Dynamic SQL and ODBC (Open Database Connectivity) both enable database access, but they serve different purposes and are used in different contexts. Here’s an overview of when to use each, along with their advantages.

1. Dynamic SQL

Dynamic SQL involves constructing and executing SQL statements at runtime within a program, allowing the SQL queries to be flexible and adaptable based on the needs of the application or user inputs.

When to Use Dynamic SQL

  • Ad-hoc Queries: When the SQL statements are not known until runtime and need to be generated dynamically based on user input, filters, or other variables.
  • Flexible Reporting and Analytics: For applications that support user-driven queries, such as reporting tools where users can specify different fields, filters, and sorting options.
  • Variable Schema: When the database schema may change frequently, dynamic SQL allows adapting the queries to match the updated schema without recompiling the entire application.
  • Conditional Queries: When queries vary greatly based on different conditions, dynamic SQL allows for conditional logic within the query.

Advantages of Dynamic SQL

  • Flexibility: Dynamic SQL allows the construction of SQL statements on the fly, making it suitable for applications with varying query requirements.
  • Adaptability to Schema Changes: The flexibility to change SQL queries at runtime without modifying the underlying program code.
  • Enhanced User Control: Users can specify custom queries, filters, and sort orders, providing greater control over the data.

However, dynamic SQL can pose security risks (like SQL injection) if not handled carefully, and it may have performance overhead due to parsing and compiling the SQL at runtime.

2. ODBC (Open Database Connectivity)

ODBC is a standardized API that allows an application to connect to different database systems (e.g., MySQL, PostgreSQL, SQL Server) through a common interface, regardless of the underlying database.

When to Use ODBC

  • Database Independence: When you need an application to connect to multiple types of databases without changing the application code.
  • Standardized Database Access: When working in a multi-database environment or in scenarios where the backend database might change.
  • Legacy or Cross-Platform Systems: When building applications that need to connect to different types of databases on different platforms, such as legacy systems or third-party databases.

Advantages of ODBC

  • Database Agnostic: Allows an application to interact with different databases without rewriting the code for each database system.
  • Consistency: Provides a consistent API for database operations, making code more portable and maintainable.
  • Connection Pooling: Many ODBC drivers support connection pooling, which can improve performance by reusing database connections rather than creating a new connection for each query.
  • Error Handling and Diagnostics: ODBC provides standardized error handling and diagnostic functions, which can simplify troubleshooting.

Choosing Between Dynamic SQL and ODBC

Criteria Dynamic SQL ODBC
Primary Purpose Flexible, runtime-constructed queries Database-independent connectivity
Use Case Ad-hoc and user-defined queries Multi-database, cross-platform applications
Flexibility High, as queries are built dynamically Moderate, due to standardized query syntax
Database Specificity Typically tied to a single database Supports multiple database systems
Performance Lower for complex dynamic queries Generally higher, with support for connection pooling
Security Needs careful handling (risk of SQL injection) ODBC APIs handle parameterized queries safely

Summary

  • Use Dynamic SQL when you need flexible, runtime-generated SQL queries, typically in cases where queries vary significantly based on user input or conditions.
  • Use ODBC when you need a database-agnostic solution, especially for applications that must connect to various databases through a consistent interface.

Each approach offers unique advantages, with dynamic SQL providing greater query flexibility and ODBC enabling cross-database compatibility.