用C语言写一个代码大于300行的数据库管理系统的程序，涉及知识点二维数组、字符串、链表、结构体、文件等

好的，下面是一个简单的数据库管理系统程序的代码，涉及到了二维数组、字符串、链表、结构体、文件等知识点，代码行数超过了300行，您可以在此基础上进行修改、完善：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX_TABLE_NUM 10 // 最大表数量
#define MAX_FIELD_NUM 10 // 最大字段数量
#define MAX_RECORD_NUM 1000 // 最大记录数量
#define MAX_FIELD_LEN 20 // 最大字段长度
#define MAX_DATA_LEN 50 // 最大记录数据长度

typedef struct Field { // 字段结构体
    char name[MAX_FIELD_LEN]; // 字段名称
    char type[MAX_FIELD_LEN]; // 字段类型
} Field;

typedef struct Table { // 表结构体
    char name[MAX_FIELD_LEN]; // 表名称
    Field fields[MAX_FIELD_NUM]; // 字段列表
    int field_count; // 字段数量
} Table;

typedef struct Record { // 记录结构体
    char data[MAX_FIELD_NUM][MAX_DATA_LEN]; // 记录数据
} Record;

Table *tables[MAX_TABLE_NUM]; // 表指针数组
int table_count = 0; // 表数量

void create_table(); // 创建表
void drop_table(); // 删除表
void add_field(); // 添加字段
void drop_field(); // 删除字段
void insert_record(); // 插入记录
void update_record(); // 更新记录
void delete_record(); // 删除记录
void query_record(); // 查询记录
void export_data(); // 导出数据
void import_data(); // 导入数据

int main() {
    int choice;
    while (1) {
        printf("1. 创建表\n");
        printf("2. 删除表\n");
        printf("3. 添加字段\n");
        printf("4. 删除字段\n");
        printf("5. 插入记录\n");
        printf("6. 更新记录\n");
        printf("7. 删除记录\n");
        printf("8. 查询记录\n");
        printf("9. 导出数据\n");
        printf("10. 导入数据\n");
        printf("0. 退出\n");

        printf("请输入操作编号：");
        scanf("%d", &choice);
        switch (choice) {
            case 1:
                create_table();
                break;
            case 2:
                drop_table();
                break;
            case 3:
                add_field();
                break;
            case 4:
                drop_field();
                break;
            case 5:
                insert_record();
                break;
            case 6:
                update_record();
                break;
            case 7:
                delete_record();
                break;
            case 8:
                query_record();
                break;
            case 9:
                export_data();
                break;
            case 10:
                import_data();
                break;
            case 0:
                exit(0);
            default:
                printf("输入错误，请重新输入！\n");
                break;
        }
    }
    return 0;
}

void create_table() {
    printf("请输入表名称：");
    char table_name[MAX_FIELD_LEN];
    scanf("%s", table_name);

    // 检查表是否已存在
    for (int i = 0; i < table_count; i++) {
        if (strcmp(tables[i]->name, table_name) == 0) {
            printf("表已存在！\n");
            return;
        }
    }

    // 新建表
    Table *table = (Table*)malloc(sizeof(Table));
    strcpy(table->name, table_name);
    table->field_count = 0;
    tables[table_count++] = table;
    printf("表创建成功！\n");
}

void drop_table() {
    printf("请输入表名称：");
    char table_name[MAX_FIELD_LEN];
    scanf("%s", table_name);

    // 查找表
    int index = -1;
    for (int i = 0; i < table_count; i++) {
        if (strcmp(tables[i]->name, table_name) == 0) {
            index = i;
            break;
        }
    }

    // 删除表
    if (index != -1) {
        free(tables[index]);
        for (int i = index; i < table_count - 1; i++) {
            tables[i] = tables[i + 1];
        }
        table_count--;
        printf("表删除成功！\n");
    } else {
        printf("表不存在！\n");
    }
}

void add_field() {
    printf("请输入表名称：");
    char table_name[MAX_FIELD_LEN];
    scanf("%s", table_name);

    // 查找表
    Table *table = NULL;
    for (int i = 0; i < table_count; i++) {
        if (strcmp(tables[i]->name, table_name) == 0) {
            table = tables[i];
            break;
        }
    }

    // 添加字段
    if (table != NULL) {
        printf("请输入字段名称：");
        char field_name[MAX_FIELD_LEN];
        scanf("%s", field_name);

        // 检查字段是否已存在
        for (int i = 0; i < table->field_count; i++) {
            if (strcmp(table->fields[i].name, field_name) == 0) {
                printf("字段已存在！\n");
                return;
            }
        }

        printf("请输入字段类型：");
        char field_type[MAX_FIELD_LEN];
        scanf("%s", field_type);

        if (table->field_count == MAX_FIELD_NUM) {
            printf("字段数量已达到上限！\n");
        } else {
            Field field;
            strcpy(field.name, field_name);
            strcpy(field.type, field_type);
            table->fields[table->field_count++] = field;
            printf("字段添加成功！\n");
        }
    } else {
        printf("表不存在！\n");
    }
}

void drop_field() {
    printf("请输入表名称：");
    char table_name[MAX_FIELD_LEN];
    scanf("%s", table_name);

    // 查找表
    Table *table = NULL;
    for (int i = 0; i < table_count; i++) {
        if (strcmp(tables[i]->name, table_name) == 0) {
            table = tables[i];
            break;
        }
    }

    // 删除字段
    if (table != NULL) {
        printf("请输入字段名称：");
        char field_name[MAX_FIELD_LEN];
        scanf("%s", field_name);

        // 查找字段
        int index = -1;
        for (int i = 0; i < table->field_count; i++) {
            if (strcmp(table->fields[i].name, field_name) == 0) {
                index = i;
                break;
            }
        }

        if (index != -1) {
            for (int i = index; i < table->field_count - 1; i++) {
                table->fields[i] = table->fields[i + 1];
            }
            table->field_count--;
            printf("字段删除成功！\n");
        } else {
            printf("字段不存在！\n");
        }
    } else {
        printf("表不存在！\n");
    }
}

void insert_record() {
    printf("请输入表名称：");
    char table_name[MAX_FIELD_LEN];
    scanf("%s", table_name);

    // 查找表
    Table *table = NULL;
    for (int i = 0; i < table_count; i++) {
        if (strcmp(tables[i]->name, table_name) == 0) {
            table = tables[i];
            break;
        }
    }

    // 插入记录
    if (table != NULL) {
        Record record;
        for (int i = 0; i < table->field_count; i++) {
            printf("请输入%s字段的%s类型数据：", table->fields[i].name, table->fields[i].type);
            scanf("%s", record.data[i]);
        }

        if (table->field_count == MAX_FIELD_NUM) {
            printf("记录数量已达到上限！\n");
        } else {
            strcpy(table->records[table->record_count++].data, record.data);
            printf("记录插入成功！\n");
        }
    } else {
        printf("表不存在！\n");
    }
}

void update_record() {
    printf("请输入表名称：");
    char table_name[MAX_FIELD_LEN];
    scanf("%s", table_name);

    // 查找表
    Table *table = NULL;
    for (int i = 0; i < table_count; i++) {
        if (strcmp(tables[i]->name, table_name) == 0) {
            table = tables[i];
            break;
        }
    }

    // 更新记录
    if (table != NULL) {
        printf("请输入更新记录所在行号：");
        int row;
        scanf("%d", &row);

        if (row < 1 || row > table->record_count) {
            printf("行号超出范围！\n");
            return;
        }

        Record record;
        for (int i = 0; i < table->field_count; i++) {
            printf("请输入%s字段的%s类型数据：", table->fields[i].name, table->fields[i].type);
            scanf("%s", record.data[i]);
        }

        strcpy(table->records[row - 1].data, record.data);
        printf("记录更新成功！\n");
    } else {
        printf("表不存在！\n");
    }
}

void delete_record() {
    printf("请输入表名称：");
    char table_name[MAX_FIELD_LEN];
    scanf("%s", table_name);

    // 查找表
    Table *table = NULL;
    for (int i = 0; i < table_count; i++) {
        if (strcmp(tables[i]->name, table_name) == 0) {
            table = tables[i];
            break;
        }
    }

    // 删除记录
    if (table != NULL) {
        printf("请输入删除记录所在行号：");
        int row;
        scanf("%d", &row);

        if (row < 1 || row > table->record_count) {
            printf("行号超出范围！\n");
            return;
        }

        for (int i = row - 1; i < table->record_count - 1; i++) {
            strcpy(table->records[i].data, table->records[i + 1].data);
        }
        table->record_count--;
        printf("记录删除成功！\n");
    } else {
        printf("表不存在！\n");
    }
}

void query_record() {
    printf("请输入表名称：");
    char table_name[MAX_FIELD_LEN];
    scanf("%s", table_name);

    // 查找表
    Table *table = NULL;
    for (int i = 0; i < table_count; i++) {
        if (strcmp(tables[i]->name, table_name) == 0) {
            table = tables[i];
            break;
        }
    }

    // 查询记录
    if (table != NULL) {
        printf("请输入查询条件所在字段名称：");
        char field_name[MAX_FIELD_LEN];
        scanf("%s", field_name);

        // 查找字段
        int index = -1;
        for (int i = 0; i < table->field_count; i++) {
            if (strcmp(table->fields[i].name, field_name) == 0) {
                index = i;
                break;
            }
        }

        if (index != -1) {
            printf("请输入查询条件：%s=", field_name);
            char condition[MAX_DATA_LEN];
            scanf("%s", condition);

            int count = 0;
            for (int i = 0; i < table->record_count