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# select()

Here’s how to use select() functions.

## A. select()

Description:

Return members of a sequence which satisfy a condition.

Syntax:

 A.select(x) A.select(x1:y1, x2:y2, ......xi:yi) The simplified syntax of "&&" for a multiple-condition query, which is equal to A.select(x1== y1 && x2== y2 &&...... xi==yi)

Note:

The function computes expression x against each member of the sequence A, and returns a new sequence composed of those members which make x return true. When there are no parameters, it returns all the members. Note that if the name of a to-be-referenced column of the sequence is the same as a cell name, the sequence name should be attached before the column name in the expression.

Parameter:

 A A sequence x A Boolean expression, which can be null xi:yi xi is an expression, and yi is a comparing value

Option:

 @1 Return the first member that fulfills the condition @z Search the members from back to front @b Use binary search, which requires that A is an ascendingly ordered sequence and that parameters are separated by colon or they are expressions returning numeric values; the eligible members are found when result of expressions are 0 @m Use parallel algorithm to handle data-intensive or computation-intensive tasks; no definite order for the records in the result set; it can’t be used with @1bz options @t Return an empty table sequence with data structure if the grouping and aggregate operation over the sequence returns null @c Enable getting the eligible member(s) from left to right beginning from the first member until the first ineligible member appears @r Enable searching for the first eligible members from left to right and getting all members after it (inclusive) until the last one @v Return result as a pure table sequence when sequence A is a pure table sequence; return a pure sequence when this option is absent

Return value:

A sequence or a table sequence

Example:

 A 1 [2,5,4,3,2,1,4,1,3] 2 =A1.select(~>3) [5,4,4]. 3 =A1.select@1(~>3) 5; return the first eligible member. 4 =demo.query("select EID,NAME,GENDER,DEPT,SALARY  from EMPLOYEE order by EID") 5 =A4.select(GENDER:"F",SALARY:16000) Multi-condition query.

.

Specify search direction:

 A 1 [2,5,4,3,2,1,4,1,3] 2 =A1.select(~>3) [5,4,4]. 3 =A1.select@z(~>3) [4,4,5]; search backwards. 4 [8,10,3,5,7,9,11,13,7] 5 =A4.select@c(~>7) Return [8,10]; search from the first member until the first ineligible member appears. 6 =A4.select@zc(~>6) Return [7,13,11,9,7]; search backwards until the first ineligible member appears. 7 =A4.select@r(~>10) Return [11,13,7]; Search for the first eligible member from the first member in order and get all members after it (inclusive).

High-efficiency search and filtering:

 A 1 =demo.query("select EID,NAME,GENDER,DEPT,BIRTHDAY from employee") 2 =A1.select@m(GENDER=="F") @m option enables parallel computation when there is a large volume of data to increase performance. 3 =A1.sort(EID) Sort A1 by EID in the ascending order. 4 =A3.select@b(EID<10) As A3 is an ascending sequence, here we use binary search to perform the query, for which parameter x should be a numeric expression.

When a column name and a cell name are same:

 A 1 =to(3).new(~:ID,~*~:A1) 2 =A1.select(A1.A1==4) As the column name and the cell name are same, the column name should be suffixed by sequence name when it is referenced in an expression.

Return a pure table sequence:

 A 1 =demo.query("select EID,NAME,GENDER,DEPT,BIRTHDAY from employee").keys(EID) 2 =A1.i() Convert table sequence A1 to a pure table sequence. 3 =A2.select(GENDER=="M") Return a pure sequence. 4 =A2.select@v(GENDER=="F") @v option enables returning a pure table sequence.

Related function:

Description:

Return members of a table sequence which satisfy a condition.

Syntax:

T.select@i(x)

Note:

The function computes expression x against each member of table sequence T, and returns a new table sequence composed of those members which make x return true

T is a table sequence for which an index is already created. T.select@i() will reuse the table sequence’s index and order of records in the result set will probably be disrupted.

The function returns all members when parameter x is absent.

Parameter:

 T A table sequence for which an index is already created x Filtering expression; can be null

Return value:

A table sequence

Example:

 A 1 =demo.query("select * from DEPT").keys@i(DEPTID) Return a table sequence whose key is DEPTID and create the hash index for it. 2 =A1.select@i(FATHER==12) Reuse the index created in A1 and return the record where FATHER is 12 in A1’s table sequence.

## T.select()

Description:

Define a record filtering operation on a pseudo table and return a new pseudo table.

Syntax:

T.select(x)

Note:

The function defines a computation on pseudo table T, which will calculate expression x on each of its records and get records that make x true, and return a new pseudo table.

It returns a pseudo table retaining all records when parameter x is absent.

Parameter:

 T A pseudo table x A Boolean expression that is a filter condition, which can be null

Return value:

Pseudo table object

Example:

 A 1 =create(file).record(["D:/file/pseudo/app.ctx"]) Below is data in composite table app.ctx: 2 =pseudo(A1) Generate a pseudo table object. 3 =A2.select(eid>7) Define a computation on A2’s pseudo table, which will get records that satisfy eid>7, and return a new pseudo table. 4 =A3.import() Import data from A3’s pseudo table while executing the computation defined in A3 on A2’s pseudo table, and return the following table: 5 =A2.select() Define a computation on A2’s pseudo table, which will get all records, and return a new pseudo table. 6 =A5.import() Import data from A5’s pseudo table while executing the computation defined in A5 on A2’s pseudo table, and return the following table:

## T.select(x;f)

Description:

Define a record filtering operation on a pseudo table, return a new pseudo table, and write ineligible records to a bin file.

Syntax:

T.select(x;f)

Note:

The function defines a computation on pseudo table T, which will compute expression x on each of its records, get records that make x return true and return them to a new pseudo table, and writes records that make x return false into bin file f.

Parameter:

 T A pseudo table x A Boolean expression f A bin file

Return value:

Pseudo table

Example:

 A 1 =create(file).record(["EMPLOYEE.ctx"]) 2 =pseudo(A1) Generate a pseudo table from a composite table. 3 =file("emp_NOTHR.btx") 4 =A2.select(DEPT=="HR";A3) Define a computation on A2’s pseudo table, which will get records that meet the condition DEPT=="HR" and return them to a new pseudo table, and write records not meeting the condition into bin file emp_NOTHR.btx in the main directory. 5 =A4.import() Import data from A4’s pseudo table while executing the computation defined in A4 on A2’s pseudo table, and generate bin file emp_NOTHR.btx. Below is the data A5 returns:

## ch. select()

Description:

Attach a record filtering computation to a channel and return the orignal channel.

Syntax:

ch.select(x)

Note:

The function attaches a computation to channel ch, which will calculate expression x on each of its records and get records that make values of x true, and returns the original channel ch.

When parameter x is absent, get all records.

This is an attachment function.

Parameter:

 ch A channel x Boolean expression

Return value:

Channel

Example:

 A 1 =demo.cursor("select * from SALES") 2 =channel() Create a channel. 3 =A2.select(ORDERID>100) Attach a computation to A2’s channel, which will get records meeting the condition ORDERID>100, and return the original channel. 4 =A2.fetch() Fetch and keep the current data in the channel. 5 =A1.push(A2) Be ready to push data in A1’s cursor to A2’s channel. 6 =A1.fetch() Fetch data from cursor A1 while pushing data to channel A2 to execute the attached computation and keep the result. 7 =A2.result() Get result from the channel.

## ch.select(x,ch’ )

Description:

Send records in a channel that can’t meet the given condition into another channel.

Syntax:

ch.select(x,ch’)

Note:

The function calculates expression x against each of the records in channel ch, and sends those records over which the value of x is false into channel ch’.

Parameter:

 ch Channel x A Boolean expression ch’ Channel

Return value:

Channel

Example:

 A 1 =demo.cursor("select EID,NAME,SALARY from EMPLOYEE " ) 2 =channel() Create a channel. 3 =channel() Create a channel. 4 =A1.push(A2) Push data in A1’s cursor into A2’s channel. 5 =A2.select(EID<5,A3) Push records in A2’s channel that can’t meet the condition EID<5 into A3’s channel. 6 =A2.fetch() Attach ch.fetch() function that gets the final result set to A2’s channel to fetch and store the existing data in A2’s channel. 7 =A3.fetch() Attach ch.fetch() function that gets the final result set to A3’s channel to fetch and store the existing data in A3’s channel. 8 =A1.skip() It is when fetching data from A1’s cursor begins that data in the cursor is truly pushed to the channel and computations start. 9 =A2.result() Get result of the computation in channel A2. 10 =A3.result() Get result of the computation in channel A3.

## cs .select()

Description:

Attach the record filtering action to a cursor and return the original cursor.

Syntax:

cs.select(x)

Note:

The function attaches a computation to cursor cs, which will calculate expression x against each of the records in cursor cs and get records that make value of x true, and return the original cursor cs.

When parameter x is omitted, get all records of cursor cs. The function supports multicursors.

This is a delayed function.

Parameter:

 cs A cursor x A Boolean value

Return value:

Cursor

Example:

 A 1 =demo.cursor("select * from SCORES") Return a cursor: 2 =A1.select(STUDENTID>10) Attach a computation to cursor A1, which will select records where STUDENTID is greater than 10 from the cursor, and return cursor A1: 3 =A1.fetch() Fetch data from cursor A1 where A2’s computation is executed (it would be better that data is fetched in batches when a large amount of data is involved):

## cs.select(x,ch’ )

Description:

Send records in a cursor that can’t meet the given condition into a channel.

Syntax:

cs.select(x,ch’)

Note:

The function calculates expression x against each of the records in cursor cs, and sends those records over which the value of x is false into channel ch’.

Parameter:

 cs A cursor x A Boolean value ch’ A channel

Return value:

Channel

Example:

 A 1 =demo.cursor("select EID,NAME,SALARY from EMPLOYEE" ) 2 =channel() Create a channel. 3 =A1.select(EID<5,A2) Push cursor A1’s records not meeting condition EID<5 into A2’s channel. 4 =A2.fetch() Fetch and store the existing data in the channel. 5 =A1.fetch() Fetch data from the filtered cursor. 6 =A2.result() Get result from the channel.

## cs.select(x;f)

Description:

Attach the record filtering action to a cursor and return the original cursor while writing records that not satisfy the specified expression to a bin file.

Syntax:

cs.select(x;f)

Note:

The function attaches a record filtering action to cursor cs, which will compute expression x over each record of cursor cs, get records that make value of x true, return the original cursor cs and write the records that do not satisfy expression x into bin file f.

This is a delayed function.

Parameter:

 cs A cursor x A Boolean expression f A bin file

Return value:

Cursor

Example:

 A 1 =demo.cursor("select * from dept") Return a cursor: 2 =file("dept.btx") Generate a bin file object. 3 =A1.select(DEPTID<5;A2) Attach a computation to cursor A1, which will get records that meet DEPTID <5, return cursor A1 and write records that cannot meet the condition to bin file dept.btx: 4 =A1.fetch() dept.btx will be really generated after data is really fetched out from cursor A1; below is data in cursor A1 where the computation is executed (it would be better that data is fetched in batches when data amount is large):