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Interface to TaQL

There exists a user and a programmer interface to TaQL. Glish functions form the user interface, while C++ functions and classes form the programmer interface.

• The main TaQL interface in Glish is formed by the query function in module table (in script table.g). The function can be used to compose and execute a TaQL command using the various (optional) arguments given to the query function. E.g.

      tab := table('mytable')
      seltab1 := tab.query ('column1 > 0')
      seltab2 := seltab1.query (query='column2>5',
                                sortlist='time',
                                columns='column1,column2',
                                name='result.tab')
  

  The first command opens the table mytable. The second command does a simple query resulting in a temporary table. That temporary table is used in the next command resulting in a persistent table. The latter function call is transformed to the TaQL command:
  SELECT column1,column2 FROM $1 WHERE column2>5
  ORDERBY time GIVING result.tab
  During execution $1 is replaced by table seltab1.
  Note that the name argument generates the GIVING part to make the result persistent.

  It is possible to embed glish variables and expressions in a TaQL command using the syntax $variable and $(expression). A variable can be a standard numeric or string scalar or vector. It can also be a table tool. An expression has to result in a numeric or string scalar or vector. E.g

      tab := table('mytable')
      coldata := tab.getcol ('col');
      colmean := sum(coldata) / len(coldata);
      seltab1 := tab.query ('col > $colmean')
      seltab2 := tab.query ('col > $(sum(coldata)/len(coldata))')
      seltab3 := tab.query ('col > mean([SELECT col from $tab])')
  

  These three queries give the same result.
  The substitution mechanism is described in more detail in the substitute functions of module utility of the User Reference Manual.
  The substitution mechanism uses the eval function in glish. As of 28-Feb-2001 eval only looks at global variables. This means that in a function one needs to create a global variable (with a unique name) if the variable is to be used in a TaQL command. The global variable should be deleted at the end of the function. The name can be made unique by using the function name as a suffix. E.g.:

      myfunc := function() {
        tab := table('mytable')
        global coldata_in_myfunc;
        coldata_in_myfunc := tab.getcol ('col');
        seltab := tab.query ('col > $(sum(coldata)/len(coldata))')
        symbol_delete ('coldata_in_myfunc');
      }
  

  The other function that can be used is tablecommand. The full TaQL command has to be given to that command. The result is a table tool. E.g.

      t := tablecommand('select from GER.MS where ANTENNA1==1');
  

• The C++ interface consists of 2 parts.

  1.

  The function tableCommand in TableParse.h can be used to execute a TaQL command. The result is a Table object. E.g.

    Table seltab1 = tableCommand
           ("select from mytable where column1>0");
    Table seltab2 = tableCommand
           ("select column1,column2 from $1 where column2>5"
            " orderby time giving result.tab", seltab1);
  

  These examples do the same as the Glish ones shown above.
  Note that in the second function call the table name $1 is replaced by the object seltab1 passed to the function.

  2.

  The other interface is a true C++ interface having the advantage that C++ variables can be used. Class Table contains functions to sort a table or to select columns or rows. When selecting rows class TableExprNode (in ExprNode.h) has to be used to build a WHERE expression which can be executed by the overloaded function operator in class Table. E.g.

    Int limit = 0;
    Table tab ("mytable");
    Table seltab = tab(tab.col("column1") > limit);
  

  does the same as the first example shown above. See classes Table, TableExprNode, and TableExprNodeSet for more information on how to construct a WHERE expression.