110 lines
6.9 KiB
Markdown
110 lines
6.9 KiB
Markdown
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# Working with databases
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The functions for working with databases apply the popular and easy-to-use [SQLite](https://www.sqlite.org/index.html) engine. The convenient feature of this engine is that the entire database is located in a single file on a user PC's hard disk.
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The functions allow for convenient creation of tables, adding data to them, performing modifications and sampling using simple SQL requests:
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- receiving trading history and quotes from any formats,
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- saving optimization and test results,
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- preparing and exchanging data with other analysis packages,
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- storing MQL5 application settings and status.
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Queries allow using [statistical](/en/docs/database#math) and [mathematical](/en/docs/database#stats) functions.
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The functions for working with databases allow you to replace the most repetitive large data array handling operations with SQL requests, so that it is often possible to use the [DatabaseExecute](/en/docs/database/databaseexecute)/[DatabasePrepare](/en/docs/database/databaseprepare) calls instead of programming complex loops and comparisons. Use the [DatabaseReadBind](/en/docs/database/databasereadbind) function to conveniently obtain query results in a ready-made structure. The function allows reading all record fields at once within a single call.
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To accelerate reading, writing and modification, a database can be opened/created in RAM with the DATABASE_OPEN_MEMORY flag, although such a database is available only to a specific application and is not shared. When working with databases located on the hard disk, bulk data inserts/changes should be wrapped in transactions using [DatabaseTransactionBegin](/en/docs/database/databasetransactionbegin)/DatabaseTransactionCommit/DatabaseTransactionRollback. This accelerates the process hundreds of times.
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To start working with the functions, read the article [SQLite: Native handling of SQL databases in MQL5](https://www.mql5.com/en/articles/7463).
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| Function | Action |
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| --- | --- |
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| DatabaseOpen | Opens or creates a database in a specified file |
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| DatabaseClose | Closes a database |
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| DatabaseImport | Imports data from a file into a table |
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| DatabaseExport | Exports a table or an SQL request execution result to a CSV file |
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| DatabasePrint | Prints a table or an SQL request execution result in the Experts journal |
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| DatabaseTableExists | Checks the presence of the table in a database |
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| DatabaseExecute | Executes a request to a specified database |
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| DatabasePrepare | Creates a handle of a request, which can then be executed using DatabaseRead() |
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| DatabaseReset | Resets a request, like after calling DatabasePrepare() |
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| DatabaseBind | Sets a parameter value in a request |
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| DatabaseBindArray | Sets an array as a parameter value |
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| DatabaseRead | Moves to the next entry as a result of a request |
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| DatabaseReadBind | Moves to the next record and reads data into the structure from it |
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| DatabaseFinalize | Removes a request created in DatabasePrepare() |
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| DatabaseTransactionBegin | Starts transaction execution |
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| DatabaseTransactionCommit | Completes transaction execution |
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| DatabaseTransactionRollback | Rolls back transactions |
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| DatabaseColumnsCount | Gets the number of fields in a request |
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| DatabaseColumnName | Gets a field name by index |
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| DatabaseColumnType | Gets a field type by index |
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| DatabaseColumnSize | Gets a field size in bytes |
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| DatabaseColumnText | Gets a field value as a string from the current record |
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| DatabaseColumnInteger | Gets the int type value from the current record |
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| DatabaseColumnLong | Gets the long type value from the current record |
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| DatabaseColumnDouble | Gets the double type value from the current record |
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| DatabaseColumnBlob | Gets a field value as an array from the current record |
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Statistical functions:
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- mode – [mode](https://en.wikipedia.org/wiki/Mode_(statistics))
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- median – [median](https://en.wikipedia.org/wiki/Median) (50th percentile)
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- percentile_25 – 25th [percentile](https://en.wikipedia.org/wiki/Quantile)
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- percentile_75
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- percentile_90
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- percentile_95
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- percentile_99
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- stddev or stddev_samp — sample standard deviation
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- stddev_pop — population standard deviation
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- variance or var_samp — sample variance
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- var_pop — population variance
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Mathematical functions
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- [acos(X)](https://sqlite.org/lang_mathfunc.html#acos) – arccosine in radians
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- [acosh(X)](https://sqlite.org/lang_mathfunc.html#acosh) – hyperbolic arccosine
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- [asin(X)](https://sqlite.org/lang_mathfunc.html#asin) – arcsine in radians
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- [asinh(X)](https://sqlite.org/lang_mathfunc.html#asinh) – hyperbolic arcsine
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- [atan(X)](https://sqlite.org/lang_mathfunc.html#atan) – arctangent in radians
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- [atan2(X,Y)](https://sqlite.org/lang_mathfunc.html#atan2) – arctangent in radians of the X/Y ratio
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- [atanh(X)](https://sqlite.org/lang_mathfunc.html#atanh) – hyperbolic arctangent
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- [ceil(X)](https://sqlite.org/lang_mathfunc.html#ceil) – rounding up to an integer
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- [ceiling(X)](https://sqlite.org/lang_mathfunc.html#ceil) – rounding up to an integer
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- [cos(X)](https://sqlite.org/lang_mathfunc.html#cos) – angle cosine in radians
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- [cosh(X)](https://sqlite.org/lang_mathfunc.html#cosh) – hyperbolic cosine
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- [degrees(X)](https://sqlite.org/lang_mathfunc.html#degrees) – convert radians into the angle
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- [exp(X)](https://sqlite.org/lang_mathfunc.html#exp) – exponent
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- [floor(X)](https://sqlite.org/lang_mathfunc.html#floor) – rounding down to an integer
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- [ln(X)](https://sqlite.org/lang_mathfunc.html#ln) – natural logarithm
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- [log(B,X)](https://sqlite.org/lang_mathfunc.html#log) – logarithm to the indicated base
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- [log(X)](https://sqlite.org/lang_mathfunc.html#log) – decimal logarithm
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- [log10(X)](https://sqlite.org/lang_mathfunc.html#log) – decimal logarithm
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- [log2(X)](https://sqlite.org/lang_mathfunc.html#log2) – logarithm to base 2
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- [mod(X,Y)](https://sqlite.org/lang_mathfunc.html#mod) – remainder of division
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- [pi()](https://sqlite.org/lang_mathfunc.html#pi) – approximate Pi
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- [pow(X,Y)](https://sqlite.org/lang_mathfunc.html#pow) – power by the indicated base
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- [power(X,Y)](https://sqlite.org/lang_mathfunc.html#pow) – power by the indicated base
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- [radians(X)](https://sqlite.org/lang_mathfunc.html#radians) – convert the angle into radians
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- [sin(X)](https://sqlite.org/lang_mathfunc.html#sin) – angle sine in radians
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- [sinh(X)](https://sqlite.org/lang_mathfunc.html#sinh) – hyperbolic sine
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- [sqrt(X)](https://sqlite.org/lang_mathfunc.html#sqrt) – square root
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- [tan(X)](https://sqlite.org/lang_mathfunc.html#tan) – angle tangent in radians
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- [tanh(X)](https://sqlite.org/lang_mathfunc.html#tanh) – hyperbolic tangent
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- [trunc(X)](https://sqlite.org/lang_mathfunc.html#trunc) – truncate to an integer closest to 0
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Example:
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```
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select
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count(*) as book_count,
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cast(avg(parent) as integer) as mean,
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cast(median(parent) as integer) as median,
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mode(parent) as mode,
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percentile_90(parent) as p90,
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percentile_95(parent) as p95,
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percentile_99(parent) as p99
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from moz_bookmarks;
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```
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