1. A computer-implemented method performed by a data processing apparatus, the method comprising: receiving two or more tables extracted from at least one or more web documents, and wherein each of the one or more web documents comprise one or more corresponding text portions that are separate from the two or more tables; comparing the headers of the two or more tables and determining that the headers of the two or more tables are semantically equivalent based on the comparison; determining, based on the headers of the two or more tables being determined as semantically equivalent, that the two or more 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 documents from which the two or more stitchable tables are extracted; extracting, from the determined corresponding text portions, at least one hidden attribute that is 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 the two or more stitchable tables and the at least one hidden attribute in a new column within the union table.
1. A computer-implemented method performed by a data processing apparatus, the method comprising: receiving two or more tables extracted from at least one or more web documents, and wherein each of the one or more web documents comprise one or more corresponding text portions that are separate from the two or more tables; comparing the headers of the two or more tables and determining that the headers of the two or more tables are semantically equivalent based on the comparison; determining, based on the headers of the two or more tables being determined as semantically equivalent, that the two or more 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 documents from which the two or more stitchable tables are extracted; extracting, from the determined corresponding text portions, at least one hidden attribute that is 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 the two or more stitchable tables and the at least one hidden attribute in a new column within the union table. 6. The computer-implemented method of claim 1 , wherein the union table comprises at least one column that includes attribute values of the hidden attributes.