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Parsing CREATE TABLE statement with Regex in Node.js

Published on: 18th Jan 2021

Updated on 10th Mar 2021

Overview

Usually, we start developing a new system with designing database and then writing codes. If you are not a fan of ORM, you will have to manually writing the code for the data objects and it is very time consuming. To avoid writing the data objects manually, we develop code generator that will generate the basic codes and then we customize it.

To develop code generator that generate data object code, the generator must first understand the table structure. There are two ways to do that.

  1. For MSSQL, we query the INFORMATION_SCHEMA view for gathering the field name, data type and it's attributes.
  2. Parse the CREATE TABLE statement.

The first option is quite easy and straight forward. The catch is that it is database dependant and might not supported in certain database engine. The second option should be more viable because of the CREATE TABLE statement is very much following the ANSI-SQL standard with some minor database dependant feature.

In this article, we will focus in parsing the CREATE TABLE statement with RegEx using JavaScript.

Ways of parsing text

Let's see how many ways of parsing text:

  1. Munching the CREATE TABLE statement character by character

    The naive way of parsing text is to write a program to check one character after another and then look for the "terminator" (i.e., a symbol that mark the end of the current section or statement). Parsing text in this way is error prone and it will take a lot efforts in guessing and testing.

  2. Using RegEx

    With RegEx, it requires to find the pattern in the text and then come out with the expression. Luckily, SQL syntax is well defined and writing the expression should be easier.

    The good thing about this method is that you are focusing on finding a better expression instead of deciding how many characters to be read.

Here it goes

We are going to develop the parser using JavaScript which runs in Node.js and it parses SQLite CREATE TABLE statement. Let's create a .js file, copy the codes and run it in Node.js.


let sql = `   create table if not exists tb_sales (
    id integer primary   key
    , dt date
    , prod_id integer   not   null
    , qty integer default (0) not null
    , unit_price   real
    , amt real 
    , doc_no text null
, cust_code text

, modified_on datetime default current_timestamp,
modified_by text
);
`;

let result;

// The regex for each component:
let rg_tb = /(create\s+table\s+if\s+not\s+exists|create\s+table)\s(?<tb>.*)\s\(/gim;
let rg_fld = /(\w+\s+text.*|\w+\s+varchar.*|\w+\s+char.*|\w+\s+real.*|\w+\s+float.*|\w+\s+integer.*|\w+\s+int.*|\w+\s+datetime.*|\w+\s+date.*)/gim;
let rg_fld2 = /(?<fname>\w+)\s+(?<ftype>\w+)(?<fattr>.*)/gi;
let rg_not_null = /not\s+null/i
let rg_pk = /primary\s+key/i
let rg_fld_def = /default\s(.+)/gi

// look for table name
result = rg_tb.exec(sql);
console.log('TABLE NAME:', result.groups.tb);

let fld_list = [];

while ((result = rg_fld.exec(sql)) != null) {
    let f = result[0];

    //reset
    rg_fld2.lastIndex = 0;
    let fld_def = rg_fld2.exec(f);

    // remove the field definition terminator.
    let attr = fld_def.groups.fattr.replace(',', '').trim();

    // look for NOT NULL.
    let nullable = !rg_not_null.test(attr);

    // remove NOT NULL.
    let attr2 = attr.replace(rg_not_null, '');

    // look for PRIMARY KEY
    let is_pk = rg_pk.test(attr2);

    // look for DEFAULT
    let def = rg_fld_def.exec(attr2);
    if (def && def.length > 0) {
        def = def[1].trim();
    }
    else {
        def = null;
    }

    // append to the arr
    fld_list.push({
        name: fld_def.groups.fname.trim(),
        type: fld_def.groups.ftype.trim(),
        nullable: nullable,
        pk: is_pk,
        def: def,
        //attr0: attr
    });
}

console.table(fld_list);

Code explains:

TABLE NAME: tb_sales
┌─────────┬───────────────┬────────────┬──────────┬───────┬─────────────────────┐
│ (index) │     name      │    type    │ nullable │  pk   │         def         │
├─────────┼───────────────┼────────────┼──────────┼───────┼─────────────────────┤
│    0    │     'id'      │ 'integer'  │   true   │ true  │        null         │
│    1    │     'dt'      │   'date'   │   true   │ false │        null         │
│    2    │   'prod_id'   │ 'integer'  │  false   │ false │        null         │
│    3    │     'qty'     │ 'integer'  │  false   │ false │        '(0)'        │
│    4    │ 'unit_price'  │   'real'   │   true   │ false │        null         │
│    5    │     'amt'     │   'real'   │   true   │ false │        null         │
│    6    │   'doc_no'    │   'text'   │   true   │ false │        null         │
│    7    │  'cust_code'  │   'text'   │   true   │ false │        null         │
│    8    │ 'modified_on' │ 'datetime' │   true   │ false │ 'current_timestamp' │
│    9    │ 'modified_by' │   'text'   │   true   │ false │        null         │
└─────────┴───────────────┴────────────┴──────────┴───────┴─────────────────────┘

So, everything looks good except that the rg_fld is not able to extract multiple fields that appears on a single line. Let's try to solve it by adjusting the expression rg_fld for more than one field definition in the same line. Consider that the paid_amt real appear next to the qty real. The code will look like this:

let sql2 = `unit_price   real
    , amt real 
, qty real, paid_amt real	 
`;

let r2;
while ((r2 = rg_fld.exec(sql2)) != null) {
    console.log('=>', r2[0])
}

And the result shown below is not what we are expecting. The problem is that qty real, paid_amt real was treated as one field instead of two fields.

=> unit_price   real
=> amt real 
=> qty real, paid_amt real

Let's tweak the expression by declaring a new one so that it look for the field definition with terminator (comma symbol) or without the terminator (i.e., the last field definition does not have terminator).

let sql2 = `unit_price   real
    , amt real 
, qty real, paid_amt real	 
`;

let r2;

let t2 = /\w+\s+real\s*,|\w+\s+real.*/gim
while ((r2 = t2.exec(sql2)) != null) {
    console.log('=>', r2[0])
}

And the result is close to what we are looking for and the comma was included in the result which we don't want.

=> unit_price   real
    ,
=> amt real 
,
=> qty real,
=> paid_amt real

Now, let's remove the comma and next line symbol manually.

let sql2 = `unit_price   real
    , amt real 
, qty real, paid_amt real	 
`;

let r2;

let t2 = /\w+\s+real\s*,|\w+\s+real.*/gim
while ((r2 = t2.exec(sql2)) != null) {
    let f = r2[0].replace('\r', '').replace('\n', '').replace(',', '');    
    console.log('=>', f)
}

The following result is clean and ready to apply rg_fld2 expression.

=> unit_price   real    
=> amt real 
=> qty real
=> paid_amt real

By the way, this parser has not been completed. You may have to figure out how to incorporate t2 expression into rg_fld.

Useful links

You will find the CREATE TABLE statement in detail in the following link,

https://sqlite.org/lang_createtable.html

There are many sites in the Internet which explains RegEx in details. Here's two of it which I find very helpful.

https://flaviocopes.com/javascript-regular-expressions/

https://javascript.info/regular-expressions

To test the regular expression that you have written, you may rely on this tool,

https://regexr.com/

Short note for RegEx

Jump to #NODEJS blog

Author

Lau