Preface

Upgrading the database is a very tedious and seemingly troublesome thing. In an interview, the interviewer asked about adding fields to update the database and upgrading it. I replied that I could make a database migration copy
Every time I judge the updated version, the interviewer says that he is puzzled. Why is it so complicated? He repeatedly asked me several times, maybe waiting for a better answer;

You need to add one or more fields to an existing table. The idea should be this way;
First, determine whether the added field exists. If it does not exist, insert it:

1

#import "FMDatabaseAdditions.h" // Import header file

// judge
if (![db columnExists:@"Fields to be added" inTableWithName:@"Table name"]){  
          
} 

2

If not, insert:

NSString *alertStr = [NSString stringWithFormat:@"ALTER TABLE %@ ADD %@ INTEGER",@"Table name",@"New field"];  
BOOL worked = [db executeUpdate:alertStr];  
if(worked){
    NSLog(@"Insert success");
}else{
    NSLog(@"Insert failure");
}

3

The upgrade sequence is as follows:

// Get Documents directory path
    NSString *documentsPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];
    // File path
    NSString *sqlitePath = [NSString stringWithFormat:@"MLChatDataBase%@.sqlite", [MLSettingTool objectForKey:ML_UserId]];
    NSString *filePath = [documentsPath stringByAppendingPathComponent:sqlitePath];

// Instantiate FMDataBase object
    _db = [FMDatabase databaseWithPath:filePath];
    [_db open];
    // Initialize data table
    NSString *userInfoSql = @"CREATE TABLE IF NOT EXISTS UserInfoData (userId VARCHAR(255) PRIMARY KEY NOT NULL, realName VARCHAR(255),headImg VARCHAR(255),mobile VARCHAR(255))";
    [_db executeUpdate:userInfoSql];

// Determine whether to include table fields
      if (![db columnExists:@"age" inTableWithName:@"UserInfoData"]){  
          NSString *alertStr = [NSString stringWithFormat:@"ALTER TABLE %@ ADD %@ INTEGER",@"UserInfoData",@"age"];  
          BOOL worked = [db executeUpdate:alertStr];  
          if(worked){
                 NSLog(@"Insert success");
          }else{
                  NSLog(@"Insert failure");
          }
          } 

     [_db close];