# 执行预备语句
此示例创建一个给定参数的预备语句，用于PQexecPrepared执行预备语句。
具体代码如下：
```
/*
 * testlibpq2.c 测试PQprepare
 */
#include <stdio.h>
#include <stdlib.h>
#include <libpq-fe.h>
#include <string.h>
int main(int argc, char * argv[])
{
    /* 此处user、passwd等变量应从环境变量或配置文件读取，环境变量需用户自己按需配置；非环境变量情况下可直接赋值字符串 */
    PGconn *conn;
    PGresult * res;
    ConnStatusType pgstatus;
    char connstr[1024];
    char cmd_sql[2048];
    int nParams = 0;
    int paramLengths[5];
    int paramFormats[5];
    Oid paramTypes[5];
    char * paramValues[5];
    int i, cnt;
    char cid[32];
    int k;
    char *passwd = getenv("EXAMPLE_PASSWD_ENV");
    char *port = getenv("EXAMPLE_PORT_ENV");
    char *hostaddr = getenv("EXAMPLE_HOST_ENV");
    char *username = getenv("EXAMPLE_USERNAME_ENV");
    char *dbname = getenv("EXAMPLE_DBNAME_ENV");
    /* PQconnectdb连接数据库, 详细的连接信息为connstr */
    sprintf(connstr,
            "hostaddr=%s dbname=%s port=%s user=%s password=%s",
             hostaddr, dbname, port, username, passwd);
    conn = PQconnectdb(connstr);
    pgstatus = PQstatus(conn);
    if (pgstatus == CONNECTION_OK)
    {
        printf("Connect database success!\n");
    }
    else
    {
        printf("Connect database fail:%s\n",PQerrorMessage(conn));
        return -1;
    }
    /* 创建表t01 */
    res = PQexec(conn, "DROP TABLE IF EXISTS t01;CREATE TABLE t01(a int, b int);INSERT INTO t01 values(1, 23);");
    if (PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        printf("Command failed: %s.\n", PQerrorMessage(conn));
        PQfinish(conn);
        return -1;
    }
    /* cmd_sql查询 */
    sprintf(cmd_sql, "SELECT b FROM t01 WHERE a = $1");
    /* cmd_sql中$1对应的参数 */
    paramTypes[0] = 23;
    /* PQprepare创建一个给定参数的预备语句 */
    res = PQprepare(conn,
                    "pre_name",
                    cmd_sql,
                    1,
                    paramTypes);
   if( PQresultStatus(res) != PGRES_COMMAND_OK )
    {
        printf("Failed to prepare SQL : %s\n: %s\n",cmd_sql, PQerrorMessage(conn));
        PQfinish(conn);
        return -1;
    }
    PQclear(res);
    paramValues[0] = cid;
    for (k=0; k<2; k++)
    {
        sprintf(cid, "%d", 1);
        paramLengths[0] = 6;
        paramFormats[0] = 0;
        /* 执行预备语句 */
        res = PQexecPrepared(conn,
                             "pre_name",
                             1,
                             paramValues,
                             paramLengths,
                             paramFormats,
                             0);
        if( (PQresultStatus(res) != PGRES_COMMAND_OK ) && (PQresultStatus(res) != PGRES_TUPLES_OK))
        {
            printf("%s\n",PQerrorMessage(conn));
            PQclear(res);
            PQfinish(conn);
            return -1;
        }
        cnt = PQntuples(res);
        printf("return %d rows\n", cnt);
        for (i=0; i<cnt; i++)
        {
            printf("row %d: %s\n", i, PQgetvalue(res, i, 0));
        }
        PQclear(res);
    }
    /* 执行结束，关闭连接 */
    PQfinish(conn);
    return 0;
}
```
示例运行结果如下：
```
Connect database success!
NOTICE:  table "t01" does not exist, skipping
return 1 rows
row 0: 23
return 1 rows
row 0: 23
```