sqlite_orm学习笔记

介绍

适用于C++的SQLite数据库的orm，https://github.com/fnc12/sqlite_orm

基本语法

创建表

表结构

struct User{
    int id;
    std::string firstName;
    std::string lastName;
    int birthDate;
    std::unique_ptr<std::string> imageUrl;
    int typeId;
};

struct UserType {
    int id;
    std::string name;
};

SQL语句

CREATE TABLE users (id integer primary key autoincrement, first_name text not null, last_name text not null, birth_date integer not null, image_url text, type_id integer not null)

CREATE TABLE user_types (id integer primary key autoincrement, name text not null DEFAULT 'name_placeholder')

sqlite_orm语句

using namespace sqlite_orm;
auto storage = make_storage("db.sqlite",
                            make_table("users",
                                       make_column("id", &User::id, autoincrement(), primary_key()),
                                       make_column("first_name", &User::firstName),
                                       make_column("last_name", &User::lastName),
                                       make_column("birth_date", &User::birthDate),
                                       make_column("image_url", &User::imageUrl),
                                       make_column("type_id", &User::typeId)),
                            make_table("user_types",
                                       make_column("id", &UserType::id, autoincrement(), primary_key()),
                                       make_column("name", &UserType::name, default_value("name_placeholder"))));

• 在创建表时不需要指明表的列的变量类型，在创建时会通过传入的参数指针进行确定
• 对于私有数据类型，可以通过创建setter和getter进行赋值和访问
• 创建表时要指明存储的文件名，如“db.sqlite" 和表名 如 “users”，“user_types”，若要存储在内存里，文件名为":memory:"或者为空
• 可为列添加的属性，如自增autoincrement(),主键 primary_key(),初始值default_value(“0”)

CRUD

插入 insert

User user{-1, "Jonh", "Doe", 664416000, std::make_unique<std::string>("url_to_heaven"), 3 };
auto insertedId = storage.insert(user);

• 返回用户id或者抛出异常
• 如果要指明id，用replace（）
  查找

try{
    auto user = storage.get<User>(insertedId);
    cout << "user = " << user.firstName << " " << user.lastName << endl;
}catch(std::system_error e) {
    cout << e.what() << endl;
}catch(...){
    cout << "unknown exeption" << endl;
}```
或者

```cpp
if(auto user = storage.get_pointer<User>(insertedId)){
    cout << "user = " << user->firstName << " " << user->lastName << endl;
}else{
    cout << "no user with id " << insertedId << endl;
}

• 第一种失败会抛出异常，第二种成功返回std::unique_ptr 失败返回nullptr

修改 updata

user.firstName = "Nicholas";
user.imageUrl = "https://cdn1.iconfinder.com/data/icons/man-icon-set/100/man_icon-21-512.png"
storage.update(user);

• 根据id修改某一行的为主属性值

storage.update_all(set(c(&User::lastName) = "Hardey",
                       c(&User::typeId) = 2),
                   where(c(&User::firstName) == "Tom"));

• 修改符合条件的行

删除 remove

storage.remove<User>(insertedId)

• 参数是id，而不是整个对象

遍历所有对象 get.all()

auto allUsers = storage.get_all<User>();
cout << "allUsers (" << allUsers.size() << "):" << endl;
for(auto &user : allUsers) {
    cout << storage.dump(user) << endl; 
}

• 默认返回一个vector容器，可以指明返回list auto allUsersList = storage.get_all();
• get.all() 内存开销较大，可以逐行遍历

for(auto &user : storage.iterate<User>()) {
    cout << storage.dump(user) << endl;
}

复杂查询

//  SELECT doctor_id
//  FROM visits
//  WHERE LENGTH(patient_name) > 8
auto selectStatement = storage.prepare(select(&Visit::doctor_id, where(length(&Visit::patient_name) > 8)));
cout << "selectStatement = " << selectStatement.sql() << endl;  //  prints "SELECT doctor_id FROM ..."
auto rows = storage.execute(selectStatement); //  rows is std::vector

//  SELECT doctor_id
//  FROM visits
//  WHERE LENGTH(patient_name) > 11
get<0>(selectStatement) = 11;
auto rows2 = storage.execute(selectStatement);

聚合函数

avg（） 平均值 .avg()
count（） 统计值 .count()
MAX（） 最大值 .max() return std::unique_ptr
MIN（） 最小值 min() return std::unique_ptr
SUM（） 求和 return std::unique_ptr
TOTAL()

条件查询

auto id5and7 = storage.get_all<User>(where(c(&User::id) <= 7 and c(&User::id) >= 5 and not (c(&User::id) == 6)));
cout << "id5and7 count = " << id5and7.size() << endl;
for(auto &user : id5and7) {
    cout << storage.dump(user) << endl;
}

• 可以使用=, !=, >, >=, <, <=, IN, BETWEEN ，LIKE，AND，OR

查询某列成员

//  SELECT id FROM users WHERE last_name = 'Doe'
auto doeIds = storage.select(&User::id, where(c(&User::lastName) == "Doe"));
cout << "doeIds count = " << doeIds.size() << endl; //  doeIds is std::vector
for(auto &doeId : doeIds) {
    cout << doeId << " ";
}
cout << endl;

查询若干列

//  `SELECT first_name, last_name FROM users WHERE id > 250 ORDER BY id`
auto partialSelect = storage.select(columns(&User::firstName, &User::lastName),
                                    where(c(&User::id) > 250),
                                    order_by(&User::id));
cout << "partialSelect count = " << partialSelect.size() << endl;
for(auto &t : partialSelect) {
    auto &firstName = std::get<0>(t);
    auto &lastName = std::get<1>(t);
    cout << firstName << " " << lastName << endl;
}

ORDER BY

//  `SELECT * FROM users WHERE id < 250 ORDER BY first_name`
auto orderedUsers2 = storage.get_all<User>(where(c(&User::id) < 250), order_by(&User::firstName));
cout << "orderedUsers2 count = " << orderedUsers2.size() << endl;
for(auto &user : orderedUsers2) {
    cout << storage.dump(user) << endl;
}