Internal Tables Part 4: READ TABLE, sy-subrc and Table Expressions

📌 KEY POINTS
✅ READ TABLE … INDEX n / WITH KEY … / WITH TABLE KEY … reads one row; check sy-subrc (0 found, 4 not found).
✅ TRANSPORTING NO FIELDS checks existence without copying data.
✅ A failed READ leaves the work area unchanged — never use it without checking sy-subrc.
💡 Modern: itab[ key = x ] raises CX_SY_ITAB_LINE_NOT_FOUND; use OPTIONAL/DEFAULT or line_exists( ).

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▶ WHERE WE ARE
Parts 1–3 introduced tables and access techniques. Reading one specific row is the most frequent operation — and a frequent source of bugs.

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▶ 1. READ BY INDEX
READ TABLE gt_flight INTO gs_flight INDEX 1.
Only for standard and sorted tables (hashed tables have no index).

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▶ 2. READ BY KEY
READ TABLE gt_flight INTO gs_flight WITH KEY carrid = 'LH' connid = '0400'.

• Any fields can be used ("free key").
• On standard tables this is a linear search unless you add BINARY SEARCH on a correctly sorted table (Part 5).
• On sorted/hashed tables, WITH TABLE KEY (complete key) uses the optimized access; WITH KEY on the leading key fields of a sorted table is also optimized.

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▶ 3. SY-SUBRC AND SY-TABIX

• sy-subrc = 0 found, 4 not found (8 with BINARY SEARCH and position beyond the end).
• sy-tabix = index of the found row (standard/sorted), useful for MODIFY … INDEX or as a start index for loops.

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▶ 4. EXISTENCE CHECK WITHOUT COPYING
READ TABLE gt_flight TRANSPORTING NO FIELDS WITH KEY carrid = 'LH'.
IF sy-subrc = 0. … ENDIF.
Modern: IF line_exists( gt_flight[ carrid = 'LH' ] ).

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▶ 5. TRANSPORTING SPECIFIC FIELDS
READ TABLE gt_flight INTO gs_flight TRANSPORTING price WITH KEY … — copies only selected fields (others in gs_flight stay as they were — clear first!).

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▶ 6. TABLE EXPRESSIONS (7.40+)

• gs_flight = gt_flight[ carrid = 'LH' connid = '0400' ].
• Not found → exception CX_SY_ITAB_LINE_NOT_FOUND (not sy-subrc!).
• Safe forms: VALUE #( gt_flight[ … ] OPTIONAL ) (initial if not found), VALUE #( gt_flight[ … ] DEFAULT ls_default ).
• Index: gt_flight[ 1 ]; field directly: gt_flight[ carrid = 'LH' ]-price.

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▶ STEP BY STEP
Run the code example: read by index, by key, check existence, and see how an unchecked failed READ keeps old data in the work area.

✅ CHECKPOINT: every READ in your code is followed by a correct success check.

An invoice program read the tax rate for each item with READ TABLE and used the work area even when the read failed. For items with a new tax code missing from the lookup table, the previous item's tax rate was applied silently. Finance found wrong tax amounts during month-end reconciliation.
The fix checked sy-subrc after every READ and raised a clear error for unknown tax codes; in later refactoring, table expressions with OPTIONAL and explicit handling made such mistakes impossible to overlook.

• Using the work area after a failed READ.
• Expecting table expressions to set sy-subrc.
• Linear READ WITH KEY on large standard tables.
• TRANSPORTING specific fields into a work area that still contains old data.
• READ INDEX on hashed tables.

• Check sy-subrc immediately after every READ TABLE.
• Use TRANSPORTING NO FIELDS or line_exists( ) for existence checks.
• Use WITH TABLE KEY on sorted/hashed tables.
• Use OPTIONAL/DEFAULT with table expressions or catch the exception.
• Clear work areas before partial TRANSPORTING reads.

Questions: "What does sy-subrc = 4 mean after READ TABLE?", "What happens to the work area if READ fails?", "How do table expressions behave when no row exists?"
Strong answer: 0 found, 4 not found; a failed READ leaves the work area unchanged, so stale data can be processed; TRANSPORTING NO FIELDS checks existence; table expressions raise CX_SY_ITAB_LINE_NOT_FOUND unless OPTIONAL or DEFAULT is used, and don't set sy-subrc; line_exists( ) is the modern existence check.