13. A computer-implemented method performed by a data processing apparatus, the method comprising: extracting two or more tables from at least one or more web pages to obtain extracted tables, and wherein each of the one or more web pages comprise one or more corresponding text portions that are separate from the two or more extracted tables; comparing the headers of the two or more extracted tables and determining that the headers of the two or more extracted tables are semantically equivalent based on the comparison; determining, based on the headers of the two or more extracted tables being determined as semantically equivalent, that the two or more extracted tables include two or more stitchable tables; determining, for each of table of the two or more stitchable tables, the corresponding text portions of the one or more web pages from which the two or more stitchable tables are extracted; extracting, from the respective determined corresponding text portions, hidden attributes that are related to content included in the two or more stitchable tables; and creating a union table by joining the two or more stitchable tables, wherein the union table comprises data from the two or more stitchable tables and the hidden attributes extracted from the web pages from which the tables in the group of stitchable tables were extracted in a new column within the union table.
13. A computer-implemented method performed by a data processing apparatus, the method comprising: extracting two or more tables from at least one or more web pages to obtain extracted tables, and wherein each of the one or more web pages comprise one or more corresponding text portions that are separate from the two or more extracted tables; comparing the headers of the two or more extracted tables and determining that the headers of the two or more extracted tables are semantically equivalent based on the comparison; determining, based on the headers of the two or more extracted tables being determined as semantically equivalent, that the two or more extracted tables include two or more stitchable tables; determining, for each of table of the two or more stitchable tables, the corresponding text portions of the one or more web pages from which the two or more stitchable tables are extracted; extracting, from the respective determined corresponding text portions, hidden attributes that are related to content included in the two or more stitchable tables; and creating a union table by joining the two or more stitchable tables, wherein the union table comprises data from the two or more stitchable tables and the hidden attributes extracted from the web pages from which the tables in the group of stitchable tables were extracted in a new column within the union table. 15. The computer-implemented method of claim 13 , wherein at least two tables were extracted from web pages on the same web site.