- Principles of database operations in MQL5
- SQL Basics
- Structure of tables: data types and restrictions
- OOP (MQL5) and SQL integration: ORM concept
- Creating, opening, and closing databases
- Executing queries without MQL5 data binding
- Checking if a table exists in the database
- Preparing bound queries: DatabasePrepare
- Deleting and resetting prepared queries
- Binding data to query parameters: DatabaseBind/Array
- Executing prepared queries: DatabaseRead/Bind
- Reading fields separately: DatabaseColumn Functions
- Examples of CRUD operations in SQLite via ORM objects
- Transactions
- Import and export of database tables
- Printing tables and SQL queries to logs
- Example of searching for a trading strategy using SQLite
Printing tables and SQL queries to logs
If necessary, an MQL program can output the contents of a table or the results of an SQL query to a log using the DatabasePrint function.
long DatabasePrint(int database, const string table_or_sql, uint flags)
The database handle is passed in the first parameter, followed by the table name or query text (table_or_sql). The SQL query must start with "SELECT" or "select", i.e. it must not change the state of the database. Otherwise, the DatabasePrint function will end with an error.
The flags parameter specifies a combination of flags that determine the formatting of the output.
- DATABASE_PRINT_NO_HEADER — do not display table column names (field names)
- DATABASE_PRINT_NO_INDEX — do not display line numbers
- DATABASE_PRINT_NO_FRAME — do not display a frame that separates the header and data
- DATABASE_PRINT_STRINGS_RIGHT — align strings to the right
If flags = 0, then columns and rows are displayed, the header and data are separated by a frame, and the rows are aligned to the left.
The function returns the number of displayed records or -1 in case of an error.
We will use the function in the next section.
Unfortunately, the function does not allow an output of prepared queries with parameters. If there are parameters, they will need to be embedded in the query text at the MQL5 level.