89 lines
3.1 KiB
Markdown
89 lines
3.1 KiB
Markdown
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# Checking if a table exists in the database
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The built-in DatabaseTableExists function allows you to check the existence of a table by its name.
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bool DatabaseTableExists(int database, const string table)
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The database descriptor and the table name are specified in the parameters. The result of the function call is true if the table exists.
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Let's extend the DBSQLite class by adding the hasTable method.
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```
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class DBSQLite
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{
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...
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bool hasTable(const string table) const
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{
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return DatabaseTableExists(handle, table);
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}
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```
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The script DBcreateTable.mq5 will check if the table has appeared.
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```
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void OnStart()
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{
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DBSQLite db(Database);
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if(db.isOpen())
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{
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PRTF(db.execute(StringFormat("CREATE TABLE %s (msg text)", Table)));
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PRTF(db.hasTable(Table));
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}
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}
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```
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Again, don't worry about potentially getting an error when trying to recreate. This does not affect the existence of the table in any way.
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```
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database error, table table1 already exists
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db.execute(StringFormat(CREATE TABLE %s (msg text),Table))=false / DATABASE_ERROR(5601)
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db.hasTable(Table)=true / ok
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```
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Since we are writing a generic helper class DBSQLite, we will provide a mechanism for deleting tables in it. SQL has the DROP command for this purpose.
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```
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class DBSQLite
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{
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...
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bool deleteTable(const string name) const
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{
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const static string query = "DROP TABLE '%s';";
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if(!DatabaseTableExists(handle, name)) return true;
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if(!DatabaseExecute(handle, StringFormat(query, name))) return false;
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return !DatabaseTableExists(handle, name)
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&& ResetLastErrorOnCondition(_LastError == DATABASE_NO_MORE_DATA);
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}
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static bool ResetLastErrorOnCondition(const bool cond)
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{
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if(cond)
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{
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ResetLastError();
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return true;
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}
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return false;
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}
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```
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Before executing the query, we check for the existence of the table and immediately exit if it does not exist.
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After executing the query, we additionally check whether the table has been deleted by calling DatabaseTableExists again. Since the absence of a table will be flagged with the DATABASE_NO_MORE_DATA error code, which is the expected result for this method, we clear the error code with ResetLastErrorOnCondition.
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It can be more efficient to use the capabilities of SQL to exclude an attempt to delete a non-existent table: just add the phrase "IF EXISTS" to the query. Therefore, the final version of the method deleteTable is simplified:
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```
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bool deleteTable(const string name) const
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{
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const static string query = "DROP TABLE IF EXISTS '%s';";
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return DatabaseExecute(handle, StringFormat(query, name));
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}
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```
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You can try to write a test script for deleting the table, but be careful not to delete a working table by mistake. Tables are deleted immediately with all data, without confirmation and without the possibility of recovery. For important projects, keep database backups.
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